Opening this early as a draft.

py37 will see its EOL in about 2 months, on 2023-06-27
https://endoflife.date/python

The changes of this PR include:

Official removal of py37 support from the package metadata and docs, as well as updates to the CI config
Tooling config updates (mypy / ruff)
Removal of compat imports
Language/grammar updates of previously annotated stuff

This will obviously be a breaking change, so ideally, other breaking changes should be included in that release.

Additional tasks after merging these changes:

Class/function signature changes with positional-only / keyword-only parameters (py38):
Perfect opportunity to fix some old issues with certain public APIs, e.g. HLSStream.parse_variant_playlist(), and more.

Everything else in regards to py38 can be done gradually over time later.
https://docs.python.org/3/whatsnew/3.8.html