[3.14] gh-123681: Check NORMALIZE_CENTURY behavior at runtime; require C99 (GH-136022) (GH-137947)

A runtime check is needed to support cross-compiling.

Remove the _Py_NORMALIZE_CENTURY macro.
Remove _pydatetime.py's _can_support_c99.
(cherry picked from commit 719e5c3f7111bcda5eee72fe648786c427c4d4c2)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>