gh-108767: Replace ctype.h functions with pyctype.h functions (#108772)

Replace <ctype.h> locale dependent functions with Python "pyctype.h"
locale independent functions:

* Replace isalpha() with Py_ISALPHA().
* Replace isdigit() with Py_ISDIGIT().
* Replace isxdigit() with Py_ISXDIGIT().
* Replace tolower() with Py_TOLOWER().

Leave Modules/_sre/sre.c unchanged, it uses locale dependent
functions on purpose.

Include explicitly <ctype.h> in _decimal.c to get isascii().