Drop 3.6

[altendky]

GitHub is raggedly allowing 3.5 and 3.6 support in Linux to drop from the setup-python action as they start using Ubuntu 22.04 instead of 20.04. actions/setup-python#544

Example failure at https://github.com/python-qt-tools/PyQt5-stubs/actions/runs/3645895301/jobs/6156457796#step:3:9.

Version 3.6.0-alpha - 3.6.X was not found in the local cache
Error: Version 3.6.0-alpha - 3.6.X with arch x64 not found
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

We could also just make our matrix ragged as well and only drop the Linux testing. Or, explicitly call out the 20.04 runners for awhile. Let me know what you think.

[The-Compiler]

Sounds great! Python 3.6 is EOL since almost a year, so yeah, let's just drop it.

[altendky]

@bluebird75, i'm happy to try to hold on a bit longer by either using 20.04 explicitly or making a ragged matrix. Just want to make sure you get a chance at input here, if you'd like.

[altendky]

I'm going to go ahead and merge this for the sake of the rest of CI being clearly good or bad. Still happy to revert and do one of the other options if anyone requests.