CI: avoid using Cython 3.0.3

[rgommers]

Closes gh-19352

Also a few very minor cleanups; combining some pip install calls and removing an install of wheel that we no longer need.

[j-bowhay]

Failure looks related

[rgommers]

Ah yes, I forgot to update environment.yml. I'll push a fix once the rest of the current CI run completes. No need to re-run CI for that I think; I checked locally that adding cython!=3.0.3 to environment.yml has the desired effect.

[rgommers]

This should be good to squash-merge now (the second commit message can be discarded).

[rgommers]

This unbreaks CI and is very straightforward, so I'll go ahead and merge it.