add venv; fix install readme

tools/Makefile

+PYTHON:= python3.7
+.PHONY: all clean
+
+all: virtualenv
+
+virtualenv:
+	test -d venv || virtualenv -p $(PYTHON) venv
+	touch venv/bin/activate
+
+clean:
+	rm -fr venv
+	find -iname "*.pyc" -delete
+