Removed empty gis models.py

Refs #22645.
Fixed #23812 -- Changed django.utils.six.moves.xrange imports to range
Updated advice on six.moves.range/xrange; refs #23812.
Ammended release notes for Refs #23969

(this includes a forward-port of 9189ef438ef885446b20bf4c017d3cf53335ab08)
Removed obsolete code for running test_loaders.py.

Running this file as a standalone script doesn't work anyway.