Restrict pytest-postgresql to < 4.0.0
Other modules still need psycopg2 and pytest-postgresql 4 introduced a hard dependency on psycopg3.
This restriction has only been needed since a recent dependency upgrade (or maybe a pip upgrade); pip has stopped being able to solve it itself for some reason.
Test Plan
the tox env creates successfully
Migrated from D7497 (view on Phabricator)