Delete python bytecode before every test run

Because python creates pyc files during tox runs, certain
changes in the tree, like deletes of files, or switching
branches, can create spurious errors.

Change-Id: I948892fb820f3d0583731627068ae3aa254121e7
Closes-Bug: #1368661

tox.ini

@@ -16,7 +16,9 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --testr-args='{posargs}'
+commands = find . -type f -name "*.pyc" -delete
+           python setup.py testr --testr-args='{posargs}'
+whitelist_externals = find
 
 [testenv:pep8]
 commands = flake8