[1.6.x] Fixed typo. Refs #21836.

Backport of 8e1fc03 from master
[1.6.x] Fixed #21880 -- Added missing items to django.utils.timezone.__all__.

Thanks Wim for the report.

Backport of 2b154ae from master.
[1.6.x] Fixed #21823 -- Upgraded six to 1.5.2

Backport of 780ae7e9f8 from master.
[1.6.x] Added missing items to 1.6.2 release notes.

Backport of 2cbe1e28fb from master
[1.6.x] Added release note stubs for 1.5.6 and 1.4.11.

Backport of dfa28981ce from master