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GH-39599: [Python] Avoid leaking references to Numpy dtypes (#39636)

### Rationale for this change

`PyArray_DescrFromScalar` returns a new reference, so we should be careful to decref it when we don't use it anymore.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* Closes: #39599

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>