[1.6.x] Added 1.6.3 release note for refs #22250.

Backport of d57ba04d89 from master
[1.6.x] Fixed #21795 -- Made add_preserved_filters account for url prefixes.

Thanks to trac username honyczek for the report. Refs #6903.

Backport of 4339e9a92d from master
[1.6.x] Replaced urllib/urlparse imports with from django.utils.six.moves.

Backport of 981b864fbd from master
[1.6.x] Fixed #22360 -- Fixed two non-deterministic tests in Python 3.4.

The order of admin's changelist filters in the querystring relied on
dict ordering.

Backport of 4d996b8e69 from master