delete python bytecode before every test run

Bring over the cleaning line from run_tests.sh for the pyc files to
all the tox runs.

This should eliminate the need to clean -x -i to kill pyc files in
your local directory to get tests to pass.

Related-Bug: #1368661

Change-Id: I00ee418eea2d82031bb510b09e63e2ec87fb1b09

tox.ini

@@ -11,7 +11,9 @@ setenv = VIRTUAL_ENV={envdir}
 
 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}'
 
 [testenv:pep8]
 commands = flake8 {posargs}