Python 3.12 support

[apoikos]

Python 3.12 is around the corner with a lot of breaking changes. Some changes have already been dealt with, but we still need to tackle at least the following removals:

• distutils: we have a single usecase in test/py/ganeti.utils.x509_unittest.py that should be easy to fix.
• asyncore: despite Drop some asyncore-related code #1701, asyncore is still in use by the http server (noded, rapi). Both of these are forking servers and asyncore was used only for signal handling. I've started work to replace the whole implementation using standard library primitives (socketserver, BaseHTTPRequestHandler).