[ENH] Deprecate numpy/math.pxd

[scoder]

Coming from #5967 (comment):

numpy/math.pxd is a relict that we should not maintain and NumPy doesn't want.

The numpy header doesn't do much special on top of libc.math, so we should first migrate all declarations in numpy/math.pxd to use libc's math.h instead, except for the few that come directly from NumPy. Cython would still include both header files if people cimport from the pxd, so it doesn't help much in terms of removing NumPy dependencies, but it would at least make it clear that users are probably better off with libc.math directly.

I'd then also add a clear C warning, as @jakirkham suggested in #5967. That seems fine for Cython 3.1, which intentionally drops old cruft and asks users to migrate.

[jakirkham]

Linking relevant PRs:

• Drop numpy/math.pxd #5967 (also linked above by Stefan)
• improve test coverage of numpy_math.pyx #6075