Use python3 as the default interpreter.

Makefile

 # Set DESTDIR if it isn't given
 DESTDIR?=/
+PYTHON?=python3
 
 .PHONY : all clean clean-build clean-pyc clean-test install test upload
 
 all :
-	python ./setup.py build
+	$(PYTHON) ./setup.py build
 
 install : all
-	python ./setup.py install --root=${DESTDIR}
+	$(PYTHON) ./setup.py install --root=${DESTDIR}
 
 clean: clean-build clean-pyc clean-test ## remove all build, test, coverage and Python artifacts
 
@@ -31,8 +32,8 @@ clean-test: ## remove test and coverage artifacts
 	rm -fr htmlcov/
 
 test :
-	python setup.py test
+	$(PYTHON) setup.py test
 	$(MAKE) -C test test
 
 upload : test
-	python ./setup.py sdist upload
+	$(PYTHON) ./setup.py sdist upload

setup.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 import sys
@@ -50,6 +50,7 @@ setup(
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Topic :: Communications',
         'Topic :: Internet',
     ],

test/lib/01-keepalive-pingreq.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a pingreq after the keepalive time
 

test/lib/01-no-clean-session.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with clean session not set.
 

test/lib/01-unpwd-empty-password-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a username and password.
 

test/lib/01-unpwd-empty-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a username and password.
 

test/lib/01-unpwd-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a username and password.
 

test/lib/01-unpwd-unicode-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a unicode username and password.
 

test/lib/01-will-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a will.
 # Will QoS=1, will retain=1.

test/lib/01-will-unpwd-set.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect with a will, username and password.
 

test/lib/01-zero-length-clientid.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client connects correctly with a zero length clientid.
 

test/lib/02-subscribe-qos0.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct SUBSCRIBE to a topic with QoS 0.
 

test/lib/02-subscribe-qos1.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct SUBSCRIBE to a topic with QoS 1.
 

test/lib/02-subscribe-qos2.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct SUBSCRIBE to a topic with QoS 2.
 

test/lib/02-unsubscribe.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct UNSUBSCRIBE packet.
 

test/lib/03-publish-b2c-qos1.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client responds correctly to a PUBLISH with QoS 1.
 

test/lib/03-publish-c2b-qos1-disconnect.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 1, then responds correctly to a disconnect.
 

test/lib/03-publish-c2b-qos2-disconnect.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 2 and responds to a disconnect.
 

test/lib/03-publish-helper-qos0.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 0.
 # Use paho.mqtt.publish helper for that.

test/lib/03-publish-helper-qos1-disconnect.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 1,
 # then responds correctly to a disconnect.

test/lib/03-publish-qos0-no-payload.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 0 and no payload.
 

test/lib/03-publish-qos0.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct PUBLISH to a topic with QoS 0.
 

test/lib/04-retain-qos0.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client sends a correct retained PUBLISH to a topic with QoS 0.
 

test/lib/08-ssl-bad-cacert.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import context
 

test/lib/08-ssl-connect-cert-auth.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect and subsequent disconnect when using SSL.
 # Client must provide a certificate.

test/lib/08-ssl-connect-no-auth.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether a client produces a correct connect and subsequent disconnect when using SSL.
 

test/lib/08-ssl-fake-cacert.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import time
 

test/lib/Makefile

 .PHONY: all test
 .NOTPARALLEL:
 
+PYTHON?=python3
+
 all :
 
 test :
-	python ./01-will-set.py python/01-will-set.test
-	python ./01-unpwd-set.py python/01-unpwd-set.test
-	python ./01-unpwd-unicode-set.py python/01-unpwd-unicode-set.test
-	python ./01-unpwd-empty-password-set.py python/01-unpwd-empty-password-set.test
-	python ./01-unpwd-empty-set.py python/01-unpwd-empty-set.test
-	python ./01-will-unpwd-set.py python/01-will-unpwd-set.test
-	python ./01-zero-length-clientid.py python/01-zero-length-clientid.test
-	python ./01-no-clean-session.py python/01-no-clean-session.test
-	python ./01-keepalive-pingreq.py python/01-keepalive-pingreq.test
-	python ./02-subscribe-qos0.py python/02-subscribe-qos0.test
-	python ./02-subscribe-qos1.py python/02-subscribe-qos1.test
-	python ./02-subscribe-qos2.py python/02-subscribe-qos2.test
-	python ./02-unsubscribe.py python/02-unsubscribe.test
-	python ./03-publish-qos0.py python/03-publish-qos0.test
-	python ./03-publish-qos0-no-payload.py python/03-publish-qos0-no-payload.test
-	python ./03-publish-c2b-qos1-disconnect.py python/03-publish-c2b-qos1-disconnect.test
-	python ./03-publish-c2b-qos2-disconnect.py python/03-publish-c2b-qos2-disconnect.test
-	python ./03-publish-b2c-qos1.py python/03-publish-b2c-qos1.test
-	python ./03-publish-helper-qos0.py python/03-publish-helper-qos0.test
-	python ./03-publish-helper-qos1-disconnect.py python/03-publish-helper-qos1-disconnect.test
-	python ./04-retain-qos0.py python/04-retain-qos0.test
-	python ./08-ssl-connect-no-auth.py python/08-ssl-connect-no-auth.test
-	python ./08-ssl-connect-cert-auth.py python/08-ssl-connect-cert-auth.test
-	python ./08-ssl-bad-cacert.py python/08-ssl-bad-cacert.test
+	$(PYTHON) ./01-will-set.py python/01-will-set.test
+	$(PYTHON) ./01-unpwd-set.py python/01-unpwd-set.test
+	$(PYTHON) ./01-unpwd-unicode-set.py python/01-unpwd-unicode-set.test
+	$(PYTHON) ./01-unpwd-empty-password-set.py python/01-unpwd-empty-password-set.test
+	$(PYTHON) ./01-unpwd-empty-set.py python/01-unpwd-empty-set.test
+	$(PYTHON) ./01-will-unpwd-set.py python/01-will-unpwd-set.test
+	$(PYTHON) ./01-zero-length-clientid.py python/01-zero-length-clientid.test
+	$(PYTHON) ./01-no-clean-session.py python/01-no-clean-session.test
+	$(PYTHON) ./01-keepalive-pingreq.py python/01-keepalive-pingreq.test
+	$(PYTHON) ./02-subscribe-qos0.py python/02-subscribe-qos0.test
+	$(PYTHON) ./02-subscribe-qos1.py python/02-subscribe-qos1.test
+	$(PYTHON) ./02-subscribe-qos2.py python/02-subscribe-qos2.test
+	$(PYTHON) ./02-unsubscribe.py python/02-unsubscribe.test
+	$(PYTHON) ./03-publish-qos0.py python/03-publish-qos0.test
+	$(PYTHON) ./03-publish-qos0-no-payload.py python/03-publish-qos0-no-payload.test
+	$(PYTHON) ./03-publish-c2b-qos1-disconnect.py python/03-publish-c2b-qos1-disconnect.test
+	$(PYTHON) ./03-publish-c2b-qos2-disconnect.py python/03-publish-c2b-qos2-disconnect.test
+	$(PYTHON) ./03-publish-b2c-qos1.py python/03-publish-b2c-qos1.test
+	$(PYTHON) ./03-publish-helper-qos0.py python/03-publish-helper-qos0.test
+	$(PYTHON) ./03-publish-helper-qos1-disconnect.py python/03-publish-helper-qos1-disconnect.test
+	$(PYTHON) ./04-retain-qos0.py python/04-retain-qos0.test
+	$(PYTHON) ./08-ssl-connect-no-auth.py python/08-ssl-connect-no-auth.test
+	$(PYTHON) ./08-ssl-connect-cert-auth.py python/08-ssl-connect-cert-auth.test
+	$(PYTHON) ./08-ssl-bad-cacert.py python/08-ssl-bad-cacert.test

test/lib/python/01-keepalive-pingreq.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/01-no-clean-session.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-unpwd-empty-password-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-unpwd-empty-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-unpwd-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-unpwd-unicode-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from __future__ import unicode_literals
 

test/lib/python/01-will-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-will-unpwd-set.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import paho.mqtt.client as mqtt
 

test/lib/python/01-zero-length-clientid.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/02-subscribe-qos0.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/02-subscribe-qos1.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/02-subscribe-qos2.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/02-unsubscribe.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-b2c-qos1.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-b2c-qos2.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-c2b-qos1-disconnect.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-c2b-qos2-disconnect.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-helper-qos0.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 
 import paho.mqtt.publish

test/lib/python/03-publish-helper-qos1-disconnect.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 
 import paho.mqtt.publish

test/lib/python/03-publish-qos0-no-payload.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/03-publish-qos0.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/04-retain-qos0.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/08-ssl-bad-cacert.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/08-ssl-connect-cert-auth.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/08-ssl-connect-no-auth.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess

test/lib/python/08-ssl-fake-cacert.test

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import subprocess