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: I9b0e7afeeb934d24c9847573cd7582709e990c9e
Closes-Bug: #1368661

tox.ini

@@ -9,7 +9,9 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands = find . -type f -name "*.pyc" -delete
+           python setup.py testr --slowest --testr-args='{posargs}'
+whitelist_externals = find
 
 [testenv:pypy]
 deps = setuptools<3.2