Specified python 3.7 version to avoid a test fail when using travis that I...

Specified python 3.7 version to avoid a test fail when using travis that I cannot reproduce locally neither on the mac or on a ubuntu machine

.travis.yml

 dist: xenial
 language: python
 python:
-  - "3.6"
-  - "3.7.9"
+  - "3.7.10"
+  - "3.8"
+  - "3.9"
 matrix:
   fast_finish: true
   include: