Fix minor typo in documentation.
Fixed #23531 -- Added CommonMiddleware.response_redirect_class.
Updated six to 1.8.0.
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.
Added "Features removed in 1.8" section to release notes.
Removed redundant link in 1.7 release notes.
Added a dummy class for HTMLParserError; refs #23763.
Fixed #23770 -- Changed serialization strategy for floats with respect to NaN and Inf

Thanks to w0rp for the report
Replaced migration state render() by apps cached property
Show migration elapsed time when verbosity>1
Passed around the state between migrations
Reused states as much as possible in migrations