Delete orphaned pyc for python3
It's easy to push a bug where we still use a python file we've deleted or renamed because we still have the pyc. To help with this our tests delete orphaned pyc files...
https://gitweb.torproject.org/stem.git/tree/stem/util/test_tools.py#n33
This doesn't presently work with python3's pycache but we can do something similar.