Support GeoNode 4.3.0

[ridoo]

The Django 4.x upgrade shipped some backwards incompatible API changes. Two exmaples are

• url vs. re_path
• removed deprecated ugettext_lazy import

Furthermore geonode-importer seems to be more strict about when a layer is considered to be valid.

Release/Tag version to support 4.2.x and update main branch to support latest Geonode version 4.3.x.

[ridoo]

Until no further answer from Mattia we just override get_valid_layers().