requirements-server: Pin psycopg2 to < 2.9
psycopg2 2.9 release broke compatibility with django 2.2 release and deposit tests are failing since.
Related discussions on github:
That error was missed by psycopg2 maintainers as they are testing against django >= 3.1 where the issue does not appear.
So pin psycopg2 version to avoid test failures.
Migrated from D5889 (view on Phabricator)