* Use the same fallback as for missing PEP-393 support.
* Prepare for "PyUnicode_READY()" and "PyUnicode_WCHAR_KIND" to be removed in Py3.12.
  See https://www.python.org/dev/peps/pep-0623/
* Avoid C compiler warnings about deprecated C-API functions in Py3.9+.

Closes https://github.com/cython/cython/issues/3925