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: Id64c86439bf265b216f7536c51fc22cea7b316da

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}'
 
 [testenv:pypy]
 deps = setuptools<3.2