Run Python 3.12 beta in CI

[Zac-HD]

Fixes #3663. Let's see how well this works...

• blocked on Python 3.12 support pytest-dev/pytest#10894
  • which is in turn waiting for 3.12b2, expected June 6th
• ignore warnings in dateutil and pytz from the deprecation of datetime.utcfromtimestamp()
• deprecation of {collections.abc,typing}.ByteString Deprecate and schedule removal of collections.abc.ByteString and typing.ByteString python/cpython#91896 - we'll ignore these too for now to preserve old behavior on old versions
• handle TypeVar objects earlier - seems sufficient for all old use-cases and we can handle the new syntax later if needed