Support modern pytest and pytest-django

[rebeccacremona]

This PR adds support for pytest-django versions 3.3.0 though 3.4.5 (the most recent available as of 1/23/19) and for pytest versions 3.6.0 through 4.1.1 (the most recent available as of 1/23/19).

It is backwards compatible.

I had to adjust the way the package requirements are specified, because pytest-django==3.3 dropped support for pytest<3.6, and it was proving very easy to get into a situation where incompatible versions of pytest and pytest-django were installed, especially when paired with the Travis cache. For example:

• requirements.txt specified pytest==3.0.4
• setup.py specified pytest-django without a version, which resolved to the latest, 3.4.5, which requires pytest>=3.6.0.
• tests failed, even though this plugin works perfectly with pytest<3.6.0 (so long as pytest-django<3.3.0 is installed) and with pytest>=3.6.0 (so long as pytest-django>=3.3.0 is installed).

By removing pytest from requirements.txt, everything just works. If a user of pytest-django-ordering has pytest-django pinned to an old version (or, for testing purposes, if we pin it to an old version in setup.py), then pip will install an old, compatible version of pytest. Otherwise, it installs more recent, also mutually-compatible versions.

I tested with a variety of combinations, including:

• pytest-django==3.2.1 paired with pytest==3.0.6
• pytest-django==3.2.1 paired with pytest==3.5.1
• pytest-django==3.30 paired with pytest==3.6.0
• pytest-django==3.4.5 paired with pytest==4.1.1
• and I think a few more in between.

Closes #3 and #4

[rlucioni]

Thank you @rebeccacremona! I'm sorry for the delay on my end, just got back from a trip. I appreciate both your contribution and the level of detail in your PR description. Changes look good to me. I'll get version 1.1.0 (since the changes are backwards compatible) up on PyPI today.

[rebeccacremona]

Awesome! Thanks!

[rlucioni]

@rebeccacremona all set! 1.1.0 is available. Thanks again for your help here.