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: I9e3b8c8cb0e6df8f3b67ec9d4fc5bc5d25356d16
Closes-Bug: #1520036

tox.ini

@@ -7,9 +7,12 @@ envlist = py27,py34,pep8
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
 install_command = pip install {opts} {packages}
+whitelist_externals = find
 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}'
 
 [tox:jenkins]
 downloadcache = ~/cache/pip