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GH-45433: [Python] Remove Cython workarounds (#45437)

### Rationale for this change

We've accumulated multiple workarounds for various Cython issues over the years.

Now that we require at least Cython 3.0, we can remove some of those.

### Are these changes tested?

Yes, by existing CI builds and tests.

### Are there any user-facing changes?

No.
* GitHub Issue: #45433

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>