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GH-41662: [Python] Ensure Buffer methods don't crash with non-CPU data (#41889)

### Rationale for this change

`hex()` and `__getitem__` currently segfault if the data is not located on the CPU.

### What changes are included in this PR?

This PR adds a check and returns `NotImplementedError` if the data is not on CPU. 

### Are these changes tested?

Yes

### Are there any user-facing changes?

No.
* GitHub Issue: #41662

Lead-authored-by: AlenkaF <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>