Migrate to CKAN2.9/py3

[etj]

CKAN extension https://github.com/geosolutions-it/ckanext-geonetwork was made for a previous version of ckan, which only supported python 2.7

All the works we're doing on CKAN since a couple of years is bc python 2 reached its end of life
CKAN 2.9 is the first version to support py3 (it supports both py2 and py3, but we're moving our extensions to py3 only)
With this change, CKAN moved from pylons to flask
This means all the extensions need to be migrated to py3/flask
There's this guide that helps a lot in upgrading https://github.com/ckan/ckan/wiki/Python-3-migration-guide-for-extensions
All in all, we need to upgrade ckanext-geonetwork to ckan2.9/py3/flask