Drop support for Django<2.2, Python<3.5, pytest<5.4?

[bluetech]

I propose we release a major version (4.0.0) which drops all of the unsupported combinations of the above.

In practice this means:

• Drop support for Django 1.8, 1.9, 1.10, 1.11, 2.0, 2.1. All of these versions are no longer supported by Django.

• Drop support for Python 2.7 and Python 3.4. No longer supported by Python and not supported by Django.

• Keep support for Python 3.5. While it's no longer supported by Python, it is supported by Django 2.2 LTS.

• Drop support for pytest 3.5 to 5.3.

Rationale:

• Reduce the huge CI matrix
• Drop compat hacks
• Be able to utilize a modern baseline

Users using Python 2 and 3.4 will automatically continue to get pytest-django 3.

Users using older Django's or pytest's will need to upgrade their version, or require pytest-django<4.

[bluetech]

Was fixed by #869. Will try to do a release soon.