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: I2d0253b0305792862111db8b1e7adc2289863ee3
Closes-Bug: #1368661

tox.ini

@@ -13,8 +13,9 @@ install_command = {toxinidir}/tools/tox_install.sh {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals = sh
-commands =
-  sh tools/pretty_tox.sh '{posargs}'
+                      find
+commands = find . -type f -name "*.py[c|o]" -delete
+           sh tools/pretty_tox.sh '{posargs}'
 
 [testenv:pep8]
 commands = flake8