CI: Unpin Cython
- [ ] closes #54756 (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.

Something is going wrong on the Cython end I think ...

Pushing this to 2.1.1. I'm gonna merge to get Cython unpinned for now. Happy to address followups in another PR. @lithomas1 - We've been pinning Cython in the ASVs; should this be unpinned too?

https://github.com/pandas-dev/pandas/blob/753d0ed38adae2acbd928c06cce9a320253fff2e/asv_bench/asv.conf.json#L45 Ah - maybe I'm confused. Looks like we're only unpinning Cython in the CI? Yeah, ASVs have to stay pinned since there was a perf regression with Cython 3 (so we'd have spurious regressions if we unpinned).