Modernize project tools

[geigerzaehler]

Modernize the tools and dependencies this project uses

• Require Python ^3.8 and beets >= 1.6
• Use Poetry
• Use black and isort
• Use Github Actions

[wisp3rwind]

This is great!

Two small points:

• I think it would be good to keep using a test matrix with a few Python versions (oldest supported, latest stable, dev). In beets itself, this has been helpful to catch some errors due to deprecations or beets' use of Python implementation details (probably not so relevant for beets-alternatives, but who knows.)
• I guess the "Add black and isort to toolchain" commit has no code changes other than applying those tools?

[wisp3rwind]

Oh, and let's keep #55 open as a reminder to actually remove all Python 2 compatibility code (in particular, the six dependency).

[geigerzaehler]

• I guess the "Add black and isort to toolchain" commit has no code changes other than applying those tools?

Yes, that’s correct.

I think it would be good to keep using a test matrix with a few Python versions

Done

Oh, and let's keep #55 open as a reminder to actually remove all Python 2 compatibility code (in particular, the six dependency).

I’ll close the issue and open a separate one to remove Python 2 compatibility code.