🚜 Initial GitHub Actions integration

[jefftriplett]

Fixes #162

This pull request:

• Adds GHA support (via Matrix)
• Adds 3.10 support
• Drops Travis
• Drops 3.5 in testing since it's well past EOL
• Drops tox
• Updates setup.cfg to make optional requirements installable

Could do:

• Delete tox.ini, but I left it in case it's useful locally.

Follow up:

• Adding DRF Matrix support since that's opinionated. 🚜 Adds DRF to Matrix #168

[fcurella]

Why wouldn't we test against DRF in all jobs? I would just add djangorestframework to the test_requires

[jefftriplett]

Why wouldn't we test against DRF in all jobs? I would just add djangorestframework to the test_requires

I noticed the old tests tested without it (see tox too for example) so I assumed we needed it. If it's not a big deal, then that checks a big ✅

[jefftriplett]

@fcurella @frankwiles do either of you see an issue dropping Python 3.5? It's been EOL since December and pip + pytest no longer support it (not that I can't use older versions).

[fcurella]

Totally fine with dropping Py 3.5

[frankwiles]

Yeah I'm fine with dropping 3.5, hell I'm ok with no longer even testing against 3.6 or at least not letting 3.6 compatibility cause us any work.

[jefftriplett]

I'm going to leave 3.6 in since it's supported through 2021. Support doesn't seem to be an issue.