tox: rm all pyc before doing unit test

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: I21ef6bf79fe5af41d3714e1d990898b267ae1fab
Closes-Bug: #1368661

tox.ini

@@ -6,11 +6,14 @@ skipsdist = True
 [testenv]
 usedevelop = True
 install_command = pip install -U {opts} {packages}
+whitelist_externals = find
 setenv =
    VIRTUAL_ENV={envdir}
 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}'
 
 [testenv:pep8]
 commands = flake8