psycopg2 is secretly required

[DanCardin]

Describe the bug
depending on pytest-mock-resources requires you to also have psycopg2 in your environment. A previous PR (I think) removed the setup.py install requirement (because not every consumer will use pg); but we still perform imports of psycopg2 which cause those imports to fail.

Expected behavior
psycopg2 should remain an optional extra, but we should guard the imports such that it allows users to not have it installed.