fix(python): use python3 postres on cent7/8

Author: noelmcloughlin

Diff for: postgres/osfamilymap.yaml

@@ -62,6 +62,10 @@ RedHat:
     gpgkey: 'https://download.postgresql.org/pub/repos/yum/RPM-GPG-KEY-PGDG-{{ release }}'
     baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/redhat/rhel-$releasever-$basearch'
 
+{%- if grains.get('osmajorrelease') >= 7) %}
+  pkg_python: python3-psycopg2
+{%- endif %}
+
 {% if repo.use_upstream_repo == true %}
   {% set data_dir = '/var/lib/pgsql/' ~ repo.version ~ '/data' %}