venv for Python setup (#653)

* Python uses venv

* Small fixes to python calls

tools/configure.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!py_virtual_env/bin/python3
 # Copyright 2020 Google LLC
 #
 # Licensed under the Apache License, Version 2.0 (the "License");