Deprecate old Python versions

[maxfischer2781]

Python 3.6 and 3.7 are deprecated. Support should be dropped to improve language feature usage and require less maintenance for backwards compatibility.

This is a collection ticket for tasks to do when dropping Python 3.6-3.7 support:

• Change back GitHub unittest.yaml workflow from runs-on: ubuntu-20.04 to runs-on: ubuntu-latest
  (added in Add CodeQL workflow for GitHub code scanning #94, see Python 3.6 x64 on Linux: Error: Version 3.6 with arch x64 not found actions/setup-python#544)
•  Investigate if _lrucache.pyi can be merged into _lrucache.py. The former was created due to runtime constraints of Py3.6.
• Investigate if typing_extensions are still needed. The prime candidate of Protocol was added in Py3.8.
• Since Py3.7, asynchronous generator expressions are valid everywhere. There is at least one occurrence in asynctools where this could simplify code.
• Use the / function parameter syntax to indicate positional-only parameters
• Declare classes as typing.final.