Update a legacy import in rb-site to use a compatibility module.
Review Request #1987 — Created Aug. 31, 2020 and updated
`rb-site upgrade` attempts to upgrade very ancient cache backend settings when found. To do this, it needs to import `parse_backend_uri()`, which Django no longer provides. However, Djblets has a compatibility import for this purpose. This is a trivial change that updates our import to use the compatibility import.