databases/postgresql1{3,4,5,6,7}-client: Recommend py-psycopg over py-psycopg2

Upstream Python package maintainers recommend using the psycopg 3.x over
psycopg2 for new projects since the latter is in maintenance mode only.

Approved by: girgen (pgsql)
Differential Revision: https://reviews.freebsd.org/D48493