Fix #583: python 3.12 compatibility

[penguinolog]

Replace as pep-0393 recommended:

• PyUnicode_AS_UNICODE by PyUnicode_ReadChar (use Stable API)

• Enable PR tests on python 3.12

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)