tox.ini / GH Actions: Remove gentoo-python3.9

[sheerluck]

After this commit most, if not all, gentoo users switched from python3.9 to python3.10 or python3.11

At the same time it became pretty hard to create python3.9-based docker images: I tried, but gentoo said "there is no python3.9 anymore, what are you doing."

I almost created a PR to fix this; still working on "Description"

[sheerluck]

A fix has been proposed in #35705.