Delete python bytecode before every test run

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

Change-Id: Ibaac514521bab11bbf552e0310d1203230c0d984
Closes-Bug: #1368661

tox.ini

@@ -10,7 +10,11 @@ setenv = VIRTUAL_ENV={envdir}
 
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --testr-args="{posargs}"
+commands = sh -c 'find . -not \( -type d -name .?\* -prune \) \
+           \( -type d -name "__pycache__" -or -type f -name "*.py[co]" \) \
+           -print0 | xargs -0 rm -rf'
+           python setup.py testr --testr-args="{posargs}"
+whitelist_externals = sh
 passenv = SWIFT_* *_proxy
 
 [testenv:pep8]