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

[michaelo]

Summary

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: jrm (mentor), otis (mentor), girgen (pgsql)

[michaelo]

Upstream:

• https://pypi.org/project/psycopg/
• https://pypi.org/project/psycopg2/

[girgen]

Approve, looks great.