gh-89653: PEP 670: Convert unicodeobject.h macros to functions (#91773)

Convert unicodeobject.h macros to static inline functions:

* PyUnicode_MAX_CHAR_VALUE()
* PyUnicode_READ()
* PyUnicode_READY()
* PyUnicode_READ_CHAR()
* PyUnicode_WRITE()

Move PyUnicode_READY() after _PyUnicode_Ready(), since it uses
_PyUnicode_Ready().

Static inline functions are wrapped by macros which casts arguments
with _PyObject_CAST() and casts 'kind' arguments to "unsigned int" to
prevent introducing new compiler warnings when passing "const
PyObject*".