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GH-39277: [Python] Fix missing byte_width attribute on DataType class (#39592)

### Rationale for this change

As mentioned in the issue, the byte_width attribute was missing on most data types, which is a small annoyance.

### What changes are included in this PR?

Add the byte_width attribute on the DataType class (which is the base class of all Arrow data types), instead of on FixedSizeBinaryType (which is a child class of DataType).

### Are these changes tested?

Yes, tests were added in `python/pyarrow/tests/test_types.py`.

### Are there any user-facing changes?

Yes, users can now access the byte_width attribute on all fixed width data types.

* Closes: #39277

Authored-by: Kevin Mingtarja <kevin.mingtarja@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>