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GH-39096: [Python] Release GIL in `.nbytes` (#39097)

### Rationale for this change

The `.nbytes` holds the GIL while computing the data size in C++, which has caused performance issues in Dask because threads were blocking each other

See #39096

### Are these changes tested?

I am not sure if additional tests are necessary here. If so, I'm happy to add them but would welcome some pointers.

### Are there any user-facing changes?

No

* Closes: #39096

Authored-by: Hendrik Makait <hendrik@makait.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>