Add Python 3.12 to the test matrix.

[tim-schilling]

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[tim-schilling]

This is failing due to python/cpython#107088 which causes <listcomp> to no longer appear in the cProfile stats. On the plus side, we can likely remove this test for future versions of python if that's the case. Though ideally we'd have a better way to tell a user that this code is a problem.

[tim-schilling]

This is blocked by psycopg/psycopg#626

We could bypass this by using psycopg

[adamchainz]

Looks like psycopg released a compatible package so the CI can be rerun.