SCons: Bump minimum Python version to 3.8

[Repiteo]

As briefly touched on in a PR bumping GHA's Ubuntu version from 20.04¹, Python 3.6 is no longer included in those future Ubuntu builds. While it's somewhat arbitrary on which version should be used for a prospective upgrade, let alone if one should happen at all, 3.8 was proposed and that feels like the safest bet to me. Both 3.6 and 3.8 are EOL, but only relatively recently for 3.8; this should still cover our bases for all supported platform baselines.

3.7/3.8 bring quite a lot of QOL goodies, such as assignment operators² and f-string self-documentation³. But by far the biggest addition comes with from __future__ import annotations, which make several issues relating to type-hints more streamlined. This even includes syntax additions that would otherwise require 3.9 or 3.10, such as:

• Optional[str] becomes str | None
• Union[str, int] becomes str | int
• List[str] becomes list[str] (and all other "simple" container types that required import before)

As such, I've added to pyproject.toml the above as a required import—a relatively common practice on modern python repos. This does make for a LOT of files changed, but 99% of that is just an extra import for python files. Anything beyond that was updating type-hints to utilize modern syntax. No functional changes were made in this PR.

Footnotes

1. https://github.com/godotengine/godot/pull/98898#issuecomment-2460153094 ↩

2. https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions ↩

3. https://docs.python.org/3/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging ↩

[Repiteo]

Superseded by #99134