Switch to psycopg3

[jeriox]

As of Django 4.2, psycopg3 is supported and support for psycopg2 will be dropped in the future. The docs suggest to check the breaking changes at https://www.psycopg.org/psycopg3/docs/basic/from_pg2.html. On the first glimpse, we shouldn't need any changes, but maybe this needs a bit more investigating