SCons: Bump minimum Python version (3.8 → 3.9)

[Repiteo]

In an upcoming SCons release, Python versions 3.7 and 3.8 are slated for deprecation. They'll still function briefly, but outright removal will likely happen soon after. This is with SCons adopting a 1-year-after-EOL minimum (discussed in their Discord). Given our Python 3.8 bump was a little over a year ago, we're arguably already utilizing the same policy, so making this our new minimum isn't breaking precedent

More pertinently, we currently have 3.9 code in our repo. #113743 inadvertantly broke compatibility with .removeprefix()¹, and this was never caught because the job running this script is not the same as the job running the minimum-supported python version. So if we opt to not bump versions yet, we'll have to change this file to use 3.8 syntax instead, which feels like more of a hassle

This has the added bonus of us no longer having to freeze our mypy version, as it's minimum-supported version is also 3.9! Consequently, this bumps the pre-commit hook for it to the latest version

Footnotes

1. https://github.com/godotengine/godot/blob/0df713417b6b7f4ba077923fde624691c3f19014/tests/compatibility_test/run_compatibility_test.py#L56 ↩

[akien-mga]

Makes sense.

I checked briefly various old Linux distros, it seems 3.9 is fine for most of them: https://pkgs.org/download/python3

• Ubuntu 22.04 LTS: 3.10
• Debian 11 (unofficial LTS until Q3 2026): 3.9
• RHEL9 and equivalents: 3.9 (we already lost support for RHEL8 a while ago as it's on 3.6)

[Calinou]

Looks good to me.