www/py-django-filter: Update to 2.1.0

• Remove optional dependencies that are not required for runtime.
• Limit the port to Python 3.4 or later because the support for Python 2 was dropped with the 2.0.0 release.

Notable changes since 1.1.0:

• Fixed a regression in FilterView introduced in 2.0. An empty QuerySet was incorrectly used whenever the FilterSet was unbound (i.e. when there were no GET parameters). The correct, pre-2.0 behaviour is now restored.
• 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/2.1.0/CHANGES.rst

Approved by: mentors (implicit)