www/py-dj21-django-filter: Update to 2.0.0

• Remove optional dependencies that are not required for runtime

Notable changes since 1.1.0:

• Drop Python 2, Django < 1.11 support
• Added Django 2.0 support
• Many other bugfixes, for a full changelog see:

https://github.com/carltongibson/django-filter/blob/2.0.0/CHANGES.rst

Approved by: miwi (mentor)
Differential Revision: https://reviews.freebsd.org/D19160