Please make `psycopg2` test dependency optional and/or support psycopg-3

[mgorny]

The psycopg2 has been superseded a while ago by psycopg >= 3. However, the most recent release make it impossible to run the test suite without psycopg2 installed:

tests/conftest.py:20: in <module>
    from psycopg2.pool import ThreadedConnectionPool
E   ModuleNotFoundError: No module named 'psycopg2'

Could you please make the dependency optional, so we could deselect the tests requiring it? Or at least move to the modern psycopg package?

[vutran1710]

Oh my bad, tks for letting me know

[vutran1710]

@mgorny can you review? #164

[vutran1710]

Resolved in #164 and #165