[PEP 695] Generate error if 3.12 type alias is called

[JukkaL]

PEP 695 type aliases raise an exception at runtime if called.

Work on #15238.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sdb9696]

Hi, fyi this PR caused an issue for me which appeared to be due to the mypy cache not having serialized python_3_12_type_alias

  File "/redacted/lib/python3.12/site-packages/mypy/nodes.py", line 261, in deserialize
    return method(data)
           ^^^^^^^^^^^^
  File "/redacted/lib/python3.12/site-packages/mypy/nodes.py", line 3699, in deserialize
    python_3_12_type_alias = data["python_3_12_type_alias"]

KeyError: 'python_3_12_type_alias'

I was able to clear this by running mypy once with --no-incremental but you might want to make the deserialization handle the case where python_3_12_type_alias is missing. Or maybe not as presumably the cache is invalidated when running a proper new release as opposed to mypy 1.11.0+dev?

[JukkaL]

The cache is normally invalidated when mypy version is changed. We don't attempt to maintain cache compatibility between mypy versions.