Drop support for EOL Python 3.9

[hugovk]

Fixes #13642.

RM for 26.1 @sbidoul is +1 on dropping 3.9: #13642 (comment)

This PR doesn't include Ruff/pyupgrade updates, per #13236.

[hugovk]

Windows failure is unrelated:

Failed in attempting to update the source: winget
The `msstore` source requires that you view the following agreements before using.
Terms of Transaction: https://aka.ms/microsoft-store-terms-of-transaction
The source requires the current machine's 2-letter geographic region to be sent to the backend service to function properly (ex. "US").

https://github.com/pypa/pip/actions/runs/24307191409/job/70970409554?pr=13905

It passed on my fork: https://github.com/hugovk/pip/actions/runs/24306974738/job/70969833125#step:5:1

Maybe restart it?

[sbidoul]

🍏

[notatallshaw]

I think there's a whole bunch of testing infrastructure around virtualenv that can be removed, but let's not overload this PR, it would be simpler if this was clean drop in support rather than follow up clean up work (I'm not going to block this PR but I think it would be cleaner to even more things like dataclass slots to it's own PR).

[hugovk]

I've dropped the slots=True commits. I'd purposefully layered the commits, so easy to do so.

Let me know if you also want the other commit dropped.