removed unused BLANK_CHOICE_NONE
Fixed memory leak in tests

The AssertNumQueriesContext didn't reset connection's use_debug_cursor
in case there was an exception. This resulted in leaking query strings
into connection.queries. Maximum memory use pre-patch was around 700MB,
post-patch it is around 200MB for Django's test suite.
Removed debug code
Updated bundled version of six.
Stopped using non-standard __globals__ and __code__ attributes.

Some alternative implementations don't have them.

Closes #19944.
Added structure in the 1.6 release notes.

The backwards-incompatible changes section wasn't structured in sections
like it is in release notes for previous versions.
Merge pull request #902 from evildmp/BLANK_CHOICE_NONE

Fixed #20043 -- Removed unused BLANK_CHOICE_NONE
Fixed #19635 -- Made fields pickleable
Fixed #19697 -- Added a deployment checklist.
Two additions to the deployment checklist.

Thanks Erik Romijn.
Minor docs fix for e11ccc76.
Fixed #19550 -- Made the filtered select widget wider in the admin.

Thanks Claude and Julien for the review.
Fixed #19510 -- Race condition in template loading.

Thanks Kronuz and regebro.
Fixed #17037 -- Added a --all option to diffsettings.