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GH-44046: [Python] Fix threading issues with borrowed refs and pandas (#44047)



### Rationale for this change

Fix threading bugs that could leads to races under the free-threaded build.

### What changes are included in this PR?

- Use `PySequence_ITEM` instead of the `Fast` variant on lists under the free-threaded build.
- Use `std::once_flag` to make sure that `pandas` staic data only gets initialized once.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #44046

Lead-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>