Bump versions and move to GitHub actions

[flaeppe]

• Remove testing with Django<2.2 and Python<3.7

  Newer versions of mypy also (incorrectly) don't accept overriding enum
  attributes when subclassing. Mitigated by temporarily removing
  attributes from Enum base class.

  Mypy ticket can be found here: Overriding non-enumeration attributes in Enum subclass fails python/mypy#12132

• Move to GitHub actions

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Based on #66 (comment)

I also tried including Django==3.2 but it failed, felt more reasonable to bump that in a separate PR.

[flaeppe]

Not sure, but could there be some configuration needed to trigger actions to run? It worked fine in my fork (without having to configure anything): https://github.com/flaeppe/django-enumfield/actions/runs/1801846479

Or could it be that my base is master?

[Swamii]

@flaeppe The actions doesn't seem to run when added from outside. For djedi-cms we merged the PR to another branch, opened a pr to master, ran the actions (which didn't even run immediately), and then merged it.

The move to getattr for the dunder properties seems fine. Is this mergable?

[flaeppe]

@Swamii Since mypy>=0.940 support for overriding dunder attributes is resolved. If we can run with that mypy version, I'd suggest we reverse the getattr-approach

[Swamii]

@Swamii Since mypy>=0.940 support for overriding dunder attributes is resolved. If we can run with that mypy version, I'd suggest we reverse the getattr-approach

Let's do that then 👍

[flaeppe]

@Swamii I've bumped the changes now. Think it's mergable, too bad with CI though. Not sure why it won't trigger. Perhaps since all github action files are new?

[Swamii]

Great let's just merge it. But last thing: changelog entry for these changes