gh-89653: PEP 670: Convert unicodeobject.h macros to functions (#91799)

Convert unicodeobject.h macros to static inline functions:

* PyUnicode_AS_DATA()
* PyUnicode_AS_UNICODE()
* PyUnicode_GET_DATA_SIZE()
* PyUnicode_GET_SIZE()

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