Delete python bytecode before every test run

Because python creates pyc files during tox runs, certain changes in the
tree, like branch switching or file deletion, can create spurious
errors.

Closes-Bug: #1368661

Change-Id: I93917b051039506d99837028700bc03337cf68f6

tox.ini

@@ -7,6 +7,7 @@ skipsdist = True
 usedevelop = True
 setenv = OS_STDOUT_NOCAPTURE=False
          OS_STDERR_NOCAPTURE=False
+         PYTHONDONTWRITEBYTECODE=1
 
 # Nowadays, TOX_CONSTRAINTS_FILE should be used, but some older scripts might
 # still be using UPPER_CONSTRAINTS_FILE, so we check both variables and use the