Use PYTHONDONTWRITEBYTECODE=1 on tox environment

This fixes bad marshal data errors after running
tests with tox and later running tests manually
via runtests.py. Fixes #1957

tox.ini

@@ -9,6 +9,8 @@ envlist =
 
 [testenv]
 commands = ./runtests.py --fast
+setenv =
+       PYTHONDONTWRITEBYTECODE=1
 
 [testenv:flake8]
 basepython = python2.7