Add Python 3.12 classifier to pyproject.toml (against 2.1.x)
Looking at https://pypi.org/project/pandas/, we were unsure if pandas supports Python 3.12 yet.
Based on multiple PRs such as https://github.com/pandas-dev/pandas/pull/56333, we assume 2.1.x also has support for Python 3.12.

(the checkboxes below should not apply for this minor doc fix?) 

- [ ] 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.

Thanks for the PR but unfortunately the 2.1.x branch is no longer accepting changes and not adding this classifier was an oversight. This will be correctly documented for version 2.2 which will be released this week. Closing