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.

Closes-Bug: #1368661

Change-Id: Idf3a46d8c95806f2d2ce28ce84ced12b625e53df
Merge "delete python bytecode before every test run"