DEPS: Drop Python 3.9
Dropping Python 3.9 support per [NEP 29](https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table) and [SPEC-0](https://scientific-python.org/specs/spec-0000/#support-window)
I think there are a few `sys.version_info` left that compare aginst 3.10, for example, in [pandas._typing](https://github.com/pandas-dev/pandas/blob/b9bfc0134455d8a5f269366f87d2a8567aa4269c/pandas/_typing.py#L95C8-L95C24). Pyupgrades's `--py310-plus` might take care of that automatically (currently set to 3.9) > Do we want to wait until we get closer to 3.0 release to merge?

I guess we could merge anytime, but I would wait a little just in case people are still using 3.9 nightlies Are people OK with doing this now?

We're coming up on the Python 3.10 drop date (end of this year) as well.

I would personally prefer dropping as close to the date specified by SPEC 0, so people aren't surprised when this happens. Since it seemed like there was approval a few months ago, I'll merge at the end of the week if there are no objections Works for me