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
Merge "delete python bytecode before every test run"