[1.7.x] Updated six to 1.8.0.

Backport of 81477c91f6 from master
[1.7.x] Removed confusing paragraph from the docs.

This snippet wasn't particularly helpful as `add_error()` is described
in greater details in the following section.

Thanks Claude Paroz for the report.

Backport of f1f0c4f16b from master
[1.7.x] Removed redundant link in 1.7 release notes.

Backport of 9f4c718b2f from master