* Don't flag attributes as unused if they are used as global variables in
another module.
* Don't consider "True" and "False" variable names.
* Abort with error message when invoked on .pyc files.
This means we can remove the whitelisted globals in run_vulture.py and
the associated xfailing test.
We also needed to adjust run_vulture.py slightly as the file attribute
got renamed to filename.
While this makes things a little more complicated and means we'll need to use
`-r` to recreate tox environments, it has several advantages:
- Full support from requires.io (including PRs)
- Workaround for https://bitbucket.org/hpk42/tox/issues/332/ so we can update
virtualenv/pip
