REL: Remove Python upper version from the release branch

While the upper version is technically correct for the released
version of NumPy (we are sure it will not work on Python 3.13)
advertising it creates some problems, mostly for locking resolvers.
They try to guess correct versions for non-released Python versions...

This is probably an ecosystem or just "reasons", but it seems less
useful than trouble to do the correct advertising here.

See numpygh-24810 for *way* too much discussion about the why (and you will
still be confused afterwards probably, so...).
This needs to be fixed or at least documented clearer upstream by
PyPA or similar, but...

Closes numpygh-24810

pyproject.toml

@@ -26,7 +26,7 @@ authors = [{name = "Travis E. Oliphant et al."}]
 maintainers = [
     {name = "NumPy Developers", email="numpy-discussion@python.org"},
 ]
-requires-python = ">=3.9,<3.13"
+requires-python = ">=3.9"
 readme = "README.md"
 classifiers = [
     'Development Status :: 5 - Production/Stable',