DEV: Upgrade Python development version to 3.9/3.10

[phofl]

We can drop Python 3.8 support soonish, but to get there we have to upgrade our development python version from 3.8. I'd prefer 3.10, but ran into a Doc problem when looking into this a couple of weeks ago (same for 3.9).

Just rebased the PR (#51133), but would be great if someone else could take a look if the doc build is still failing.

We shouldn't upgrade to 3.11, because numba isn't yet compatible with 3.11.

[twoertwein]

Is 2.1 planned to release after Apr 05, 2024? I think pandas followed numpy in the past to determine the minimum python version https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table

[phofl]

NumPy already dropped 3.8 on main, and according to their table they officially drop it next week. I don't want to drop 3.9, but I would prefer using 3.10 as development version.

[mroeschke]

Looks to have been done with #51133 so closing