Django 1.4.x is not usable with Python 3
Fix problem with testsuite modifying migrated model auth.Group

Django 1.7 comes with its own migrations framework and has bundled
migrations for all models. The test which tried to run `mptt.register()`
on auth.Group always failed because Django really does not want database
changes without migrations anymore.

We can easily substitute auth.Group with our own Group model: the test
suite only checks whether saving groups works (and saving would fail if
the database would not contain MPTT's internal fields for some reason.)
Fix all flake8 warnings but E128 and E501