ENH: Add future.python_scalars

[rhshadrach]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Adds an experimental option to return Python scalars instead of NumPy scalars across the API. This is not yet fully implemented everywhere, e.g. Series.__getitem__, but I'm hoping reductions are a substantial chunk.

This is complicated by #62988 where it was found that many of our doctests are not running. We run those doctests using NumPy>=2, and if we were to get those doctests to pass as-is, we would need to change the NumPy reprs from e.g. 2 to np.int64(2). If we then change reductions et al to returning Python scalars, we'd then change all the reprs back from e.g. np.int64(2) to 2. So instead I think we can:

• Merge this experimental option, not yet advertising it to users.
• Merge (after some work) DOC: Run all doctests #62988 where we run doctests with the experimental option enabled. This would reduce churn in the documentation.
• Finish work on this option, expose to users in pandas 3.x and start deprecation process for changing the default.
• Change default of future.python_scalars to True in 4.0, deprecate the future option.

[rhshadrach]

cc @jbrockmendel @mroeschke @jorisvandenbossche