Updated the bundled version of six to 1.4.0.
Replaced "not PY3" by "PY2", new in six 1.4.0.
Used six.moves.zip_longest, new in six 1.4.0.
Fixed #20998 -- Allow custom (de)serialization for GIS widgets

Thanks Mathieu Leplatre for the report and the initial patch.
Fixed gis test to run on non gis-enabled settings

Refs #20998.
Fixed syntax error in OSMWidget
Prevented rendering attrs to be squashed in OSMWidget
Fixed #21002 -- Documented JSON session serialization requires string keys

Thanks jeroen.pulles at redslider.net for the report.
Isolated map creation JS code
Fixed #21003 -- Ensured geometry widget return value has SRID

Thanks Mathieu Leplatre for the report and initial patch.
Fixed #20973 -- Document serving static files without django.contrib.staticfiles
Silenced deprecation warnings in SortedDict tests; refs [07876cf02b]
Removed references to django.utils.unittest which is PendingDeprecation
Silenced deprecation warning regarding old SQL location; refs #14300
Fixed #21026 -- Corrected help for manage.py test command.
Updated syncdb -> migrate in tests.
Fixed a couple unclosed file warnings in tests
Fixed "indentation is not a multiple of four" pep8 issues.
Clarified docs for some tags and filters
Fixed #21010 -- Changed ModelState to only copy _meta.local_fields.