_ctypes callbacks.c uses _Py_COMP_DIAG_IGNORE_DEPR_DECLS (#105732)

Replace #pragma with _Py_COMP_DIAG_PUSH,
_Py_COMP_DIAG_IGNORE_DEPR_DECLS and _Py_COMP_DIAG_POP to ease Python
maintenance. Also add a comment explaining why callbacks.c ignores a
deprecation warning.