Enable Python 3.12 in CI and as the main target

[nolar]

Blockers:

• aiohttp: ** 3.9 Beta testing ** aio-libs/aiohttp#7675
  • Kopf's own issue: the webhook server does not stop in the e2e test with example 17. (Caused by K8s keepalive not disconnected by the server in aiohttp's server shutdown.)
• uvloop: Python 3.12 Support for uvloop MagicStack/uvloop#567 (or we can ignore – uvloop is optional and therefore the end-users problem).

Now, aiohttp (3.8.x) fails with numerous errors like this one:

      aiohttp/_websocket.c: In function ‘__Pyx_PyIndex_AsSsize_t’:
      aiohttp/_websocket.c:3744:45: error: ‘PyLongObject’ {aka ‘struct _longobject’} has no member named ‘ob_digit’
       3744 |     const digit* digits = ((PyLongObject*)b)->ob_digit;
            |                                             ^~
      error: command '/usr/bin/gcc' failed with exit code 1