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: Ia60baa319624e8ae5f833b6956323cf9e973d1b7
Closes-Bug: #1368661

tox.ini

@@ -10,7 +10,9 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
+  find . -type f -name "*.pyc" -delete
   python setup.py testr --slowest --testr-args='{posargs}'
+whitelist_externals = find
 
 [tox:jenkins]
 downloadcache = ~/cache/pip