CI,STYLE: add spell check?
IIRC it was suggested in a comment some time ago to use a spell checker, so I thought I'd make a little PR showing how `codespell` would work (for now just applied to `pandas/core`).

Is this something we'd want? False positives can be ignored in `setup.cfg` in the `[codespell]` section
> i think we had this before but removed it

I just ran `git log -G codespell` and got nothing (other than this PR)

I did however find (with `git log -G spell`) #24299, which removed `sphinxcontrb.spelling`. That's a different tool - this one (codespell) is very fast and takes ~0.7 seconds on the whole codebase from my (non-exceptional) laptop > > i think we had this before but removed it
> 
> I just ran `git log -G codespell` and got nothing (other than this PR)
> 
> I did however find (with `git log -G spell`) #24299, which removed `sphinxcontrb.spelling`. That's a different tool - this one (codespell) is very fast and takes ~0.7 seconds on the whole codebase from my (non-exceptional) laptop

ahh right that was for the docs. okie this looks fine to me (we *may* want to even clean up the exclude list at some point as well, e.g. narrow it) but that's for other isssues. merge on green