Import coreapi directly instead of from DRF

[rpkilby]

Fixes #525.

This is ultimately an interaction between DRF+django-filter+django-guardian, which results in the following:

  File "lib/python2.7/site-packages/django_filters/compat.py", line 19, in <module>
    from rest_framework.compat import coreapi
  File "lib/python2.7/site-packages/rest_framework/compat.py", line 210, in <module>
    import guardian.shortcuts  # Fixes #1624
  File "lib/python2.7/site-packages/guardian/shortcuts.py", line 6, in <module>
    from django.contrib.auth.models import Group, Permission

Importing coreapi directly, instead of through DRF prevents the import to django-guardian.

[afiaka87]

This passes by any standard. Should be merged asap to fix bug for all the folks using django-filters with drf 3.5