CI: Change development python version to 3.10
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Mypy passes locally now. Lets see if ci passes as well.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. /preview Website preview of this PR available at: https://pandas.pydata.org/preview/51133/ cc @mroeschke any idea why it's not including the Series.dt file? It looks like it's being included in https://pandas.pydata.org/preview/51133/docs/reference/api/pandas.Series.dt.date.html? I meant https://pandas.pydata.org/docs/reference/api/pandas.Series.dt.html in our current docs. Sphinx raises a warning that the document isn't included in any toctree. the str and cat accessors work Ah hmm yeah I'm not too sure off the top of my head why it's not included. None of the sphinx versions changed with the bump right? No they are the same. This is really weird, the inheritance logic is a bit weird for the date accessor, so I think that's the problem. Would you be ok with merging anyway? I guess it shouldn't necessarily block dropping 3.8, but lets definitely raise an issue about Series.dt being missing from API reference potentially is the sphinx issue blocking this? We should address the sphinx stuff but can do as a follow up. Have to get ci green here I skipped Pypy for now. @lithomas1 any idea how to install PyPy for 3.9?

Otherwise this looks good now, but should merge the unpin matplotlib pr first. Thx, this should be good now This would need a whatsnew in 2.1 that 3.8 support has been dropped Some grepped results where we still mention 3.8

```
./doc/source/getting_started/install.rst:Officially Python 3.8, 3.9, 3.10 and 3.11.
./doc/source/development/contributing_gitpod.rst:We have built a python 3.8 environment and all development dependencies will
``` Thx, completely forgot the docs. Updated This is getting bigger and bigger (underestimated it when starting to rip out 3.8), would prefer merging and then doing the rest as follow ups Sounds good to me.
thanks @phofl 