gh-80527: Deprecate PEP 623 Unicode functions (#91801)

Deprecate functions:

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

Previously, these functions were macros and so it wasn't possible to
decorate them with Py_DEPRECATED().