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.

The target bytecodes for deletion are in normal directories,
but not in dot started directory.

Change-Id: Ie5e55b29a2f71c4c4f886de27f89f1b264b81529

tox.ini

@@ -11,6 +11,7 @@ install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
+  /usr/bin/find . -type f -name "*.py[co]" -delete
   python setup.py testr --testr-args='{posargs}'
 
 [tox:jenkins]