Regression test and patch for ticket #20924.
Combine consecutive with statements

Python 2.7 allows to combine several 'with' instructions.
Fixed #20829 -- Skip postgis metadata tables with introspection
Remove the last FIXME from the howto-release-Django doc.
Update LazyObject method proxy declarations to simpler form.
Merge pull request #1479 from nickbruun/ticket_20924

Proxy __len__ and __contains__ for LazyObject
Some code simplification
Made Model.__eq__ consider proxy models equivalent

Fixed #11892, fixed #16458, fixed #14492.
Fixed #20777 -- Admin proxy model deletion regression

Added proxy_models tests by Harm Geerts <github@geertswei.nl>.
Fixed model.__eq__ and __hash__ for no pk value cases

The __eq__ method now considers two instances without primary key value
equal only when they have same id(). The __hash__ method raises
TypeError for no primary key case.

Fixed #18864, fixed #18250