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.

requirements-test.txt

@@ -6,7 +6,7 @@ hypothesis
 pytest < 7.0.0  # v7.0.0 removed _pytest.tmpdir.TempdirFactory, which is used by some of the pytest plugins we use
 pytest-django
 pytest-mock
-pytest-postgresql
+pytest-postgresql < 4.0.0
 requests-mock != 1.9.0, != 1.9.1
 swh.core[http] >= 0.0.95
 swh.loader.git >= 0.8.0