Update test workflow

[sondrelg]

Changes

• Adds django matrix with supported package versions and python 3.9 to the test workflow 🚀
• Removes unnecessary isort config
• Updates package metadata to include python 3.9 and django 3.1

[JonasKs]

S e x y🤩 I’ll look more closely when I get home, but this all looks awesome!

[sondrelg]

It's also possible to use pre-commit for linting checks instead of black+isort+flake8, and configure a pre-commit cache.

and I was hoping to configure test to run on pushes to master or pull requests, but seems that's not possible, so think I'll revert to pull_request only. Do you agree?

[sondrelg]

All in all I think we increase the test runtime by ~30 seconds

but now there's a dedicated job for each combination of all supported python and django versions, as documented in the packages metadata + a linting job to make sure any new code is consistent with the existing code base.

[JonasKs]

Do you want to update this to use your action before merging?