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GH-43707: [Python] Fix compilation on Cython<3 (#43765)

### Rationale for this change

Fix compilation on Cython < 3

### What changes are included in this PR?

Add an explicit cast

### Are these changes tested?

N/A

### Are there any user-facing changes?

No
* GitHub Issue: #43707

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>