PGDG repo version conflicts with psycopg2

Each PGDG repo contains versions of `python-psycopg2`, `python2-psycopg2`, and `python3-psycopg2` for the version of PostgreSQL in that repo. When you `yum update` sometimes it installs a version of psycopg2 from the wrong version's repo. It should match the version of PostgreSQL you've installed, but sometimes it doesn't.

You can fix this with `yum install --disablerepo '*' --enablerepo pgdg12 python-psycopg2` (using the correct `enablerepo` for whatever version of PostgreSQL you've installed).

We should probably have the role set the `enabled` properly in the yum repo files for the desired and undesired versions of PostgreSQL.