deb: Add python-psycopg2 as a recommended dependency

Needed by dcache-star.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7091/
(cherry picked from commit efa2de674ac1ca5aa1b18d0aba186ee98c4ead22)
Merge pull request #693 from gbehrmann/fix/2.6/rb7091

deb: Add python-psycopg2 as a recommended dependency