Doesn't work w/psycopg3 for some reason, will debug later.

Improve result types for cte.select_from() by using ModelSelect

4.0.4

Fix syntax when using nulls_distinct on partial index.

Raise an `ImproperlyConfigured` instead of AttributeError if pg missing.

4.0.3

Use psycopg3 if psycopg2 is not installed.

Just some housekeeping

Update changelog, fix failing crdb sql test.

Cleanup cruft in sql and merge it into appropriate cases.

Test cleanup in prefetch

Updates to tests, gone through model_sql and cleaned-up.

First pass cleaning up models.py

Fixup some tests that were being flaky, reorg

Manual cleanup in manytomany tests.