[Python] Use SPDX convention for license metadata on PyPI

### Describe the enhancement requested

As per [PEP 639](https://peps.python.org/pep-0639/#spdx-license-expression-syntax), SPDX license expressions are now the standard for Python packages. PyPI and PyPA tools have been warning about it for a while and it is also being checked by 3rd party scanning tools (e.g. [Google's deps.dev service](https://deps.dev/pypi/pyarrow)). Additionally, putting license info into classifiers is now deprecated (although still quite common).

Does it make sense for PyArrow to start using the new standard for license metadata? Personally I think "yes", very interested to hear other people's opinions. I will raise a small PR with the change, to accompany this issue.

### Component(s)

Python