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().
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