Drop support for Python versions before 3.7, and simplify CI matrix accordingly

[zrax]

This eliminates 3 CI builds and cleans up extra conditionals for Python versions that are no longer actively supported by the PSF.

[Hoikas]

Korman still depends on Python 3.7

[dpogue]

Dropping Python 3.7 will make it difficult to build Blender+korman on Linux, because Blender 2.79 uses Python 3.7

[zrax]

Korman still depends on Python 3.7

*grumble*, I forgot to check what Blender was using (again) :/

[Hoikas]

Bumping our blender fork (yes, sigh...) to Python 3.12 (or higher) is on my todo list, but Blender's creaky dependency build system is always a major pain to deal with. 😢

[zrax]

Updated to keep 3.7, and made a comment for the next time I try to remove it :)