Fixed #23627 -- Allowed register_lookup to work as a decorator.
Fixed #23612 -- Normalized fixuture paths to allow referencing relative paths on Windows.
Avoided requiring sqlparse for a test.

Refs #23426. Thanks Markus Holtermann for the suggestion.
Converted GIS lookups to use the new Lookup API

Thanks Tim Graham, Anssi Kääriäinen and Marc Tamlyn for the
reviews.
Converted GIS lookups for Oracle
Fixed #23616 - lookup_allowed AttributError fix

If field.rel has no get_related_field(), rel_name is set to None

Thanks ranjur for reporting bug, timgraham for review,
and collinanderson for contributing tips