Fix cython3
I'm not sure if this actually works, but it gets cython to compile with py310.

- [x] closes #34014
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Should this take over from #34997? Ah, I missed #34997.  I probably should pick off the second commit into its own PR and then sort out if my attempt here is the right path or the one in the other PR.

That said, my guess is that this code is performance critical and was written the way it was as an optimization and I really don't have more than a passing understanding of the block manager so I am not the right person (and don't really have the bandwidth right now to become the right person) to take this on... Sorry for both the low-quality PR and dropping the ball on moving this forward.  I had this sitting on my machine for a while,  thought it was better to get it out in the public at all and that I would have more bandwidth to follow up :(. All good @tacaswell ! I think this is getting covered in some other PRs so closing this one to clear the queue, but effort much appreciated in any case