Cannot install psycopg2 on Python 3.8

Installing requirements currently fails with a build error for psycopg2 when using Python 3.8. This is a compatibility issue between Python 3.7 and 3.8. Solved upstream (https://github.com/psycopg/psycopg2/issues/854) in psycopg2 2.8.4. A temporary workaround is to specify Python 3.7 in the environment.