Fix the lower-requirements job: libpq-dev, psycopg 2.7
- Add libpq-dev to bindep.txt
Apparently this is needed after some unclear change.
Without this, psycopg fails to install:
"You need to install postgresql-server-dev-X.Y for building a server-side
extension or libpq-dev for building a client-side application."
- Bump the minimum version of psycopg to 2.7.2
pysycopg 2.6.2 is not able to deal with PostgreSQL 10:
"Error: could not determine PostgreSQL version from '10.6'"
Thanks to https://review.openstack.org/643231