[1.6.x] Fixed #20999 - Allow overriding formfield class with choices, without subclass restrictions.

Refs #18162. Thanks claudep and mjtamlyn for review.

Backport of 7211741fc5d50425 from master.
[1.6.x] Fixed typo in docs/ref/models/options.txt

Backport of a89c856a7a from master
[1.6.x] Fixed copy/paste error in measurement docs

Backport of e87997dd33 frmo master.
[1.6.x] Made the doc about translating string literals in templates more prominent.

Backport of 9885f07757 from master
[1.6.x] Fixed #20998 -- Allow custom (de)serialization for GIS widgets

Thanks Mathieu Leplatre for the report and the initial patch.
Backport of 102f26c92 from master.
[1.6.x] Fixed gis test to run on non gis-enabled settings

Refs #20998.
Backport of 973502c0 from master.
[1.6.x] Fixed syntax error in OSMWidget

Backport of 6ecbac21a from master.
[1.6.x] Prevented rendering attrs to be squashed in OSMWidget

Backport of b6889c68d7 from master.
[1.6.x] Fixed #21002 -- Documented JSON session serialization requires string keys

Thanks jeroen.pulles at redslider.net for the report.

Backport of 3baf1d1042 from master
[1.6.x] Isolated map creation JS code

Backport of 3550b27a8 from master.
[1.6.x] Fixed #21003 -- Ensured geometry widget return value has SRID

Thanks Mathieu Leplatre for the report and initial patch.
Backport of dd656073ad from master.
[1.6.x] Updated the bundled version of six to 1.4.0.

Backport of 42920970 from master.
[1.6.x] Replaced "not PY3" by "PY2", new in six 1.4.0.

Conflicts:
	django/db/backends/oracle/base.py
	django/db/backends/sqlite3/base.py
	django/db/models/base.py

Backport of 365c3e8b from master.
Used six.moves.zip_longest, new in six 1.4.0.
[1.6.x] Fixed #20973 -- Document serving static files without django.contrib.staticfiles

Backport of 7b04038a7f from master
[1.6.x] Fixed #21026 -- Corrected help for manage.py test command.

Backport of 8f7f8bf6884ada8622bc3696c7465c904164d314 from master.
[1.6.x] Clarified docs for some tags and filters

Backport of 1ccdc08189 from master
[1.6.x] Fixed #20958 -- Documented that GenericForeignKey fields can't be accessed in forms.

Thanks marky1991.

Backport of 533d1ab334 from master
Updated six to version 1.4.1.
[1.6.x] Tested exc_type instead of exc_value in __exit__.

exc_value might be None even though there's an exception, at least on
Python 2.6. Thanks Thomas Chaumeny for the report.

Fixed #21034.