Fixed keyerror on parent if add readonly_fields
Merge pull request #318 from robinkyc/master

Fixed keyerror if parent in readonly_fields in admin. Thanks @robinkyc
Update tox environments
Merge remote-tracking branch 'django-wiki/master'
Fix get_queryset/get_query_set horribleness.

Builds on @benjaoming's excellent PR (#317), with some minor renaming
and a backwards compatibility fix for get_queryset() support in
django 1.5.

(Fixes #316, #317)
Fix some of the failing python3 tests: Should use self._db as argument for the database in use!