[1.6.x] Updated six to 1.6.1.

Backport of 2ec82c7387 from master
[1.6.x] Fixed #21665 -- Documented that changes in assertRedirects may be required given new URL escaping behavior.

Thank pdc for the report.

Backport of 62e81bc00b from master
[1.6.x] Fixed #22247 -- Replaced "upstream" with "downstream" in cache docs.

Thanks valgarv at gmx.net for the report.

Backport of 60d2dde286 from master
[1.6.x] Fixed #22048 - Enhanced docs to cover nonexistent one-to-one relationships.

Thanks EvilDMP for the suggestion.

Backport of ec08d62a20 from master