Update importlib imports to use Python's version instead of Django's.
Review Request #8678 — Created Jan. 29, 2017 and submitted
Modern versions of Django no longer ship a backwards-compatibility module for importlib, since Python 2.7 provides this, and is the minimum version required by Django. Djblets 0.10 also requires Python 2.7, so we can safely rely on Python's version of this module, getting us a step closer to compatibility with modern versions of Django.
Unit tests pass against Django 1.6.
These failures no longer show up on unit tests against Django 1.10.