Fixed #13181 -- Added support for callable choices to forms.ChoiceField

Thanks vanschelven and expleo for the initial patch.
Removed unused import.
Added missing docs to DiscoverRunner for keepdb option; refs #20550.
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
Fixed #23785 -- Typo in docs/releases/1.6.txt
Fixed a typo in 1.6 release notes.
Fixed #23789 -- TemplateResponse handles context differently from render
Simplified MySQL storage engine detection using INFORMATION_SCHEMA.ENGINES table

Query the table introduced in MySQL 5.1; refs #23144.
Removed legacy code from old transaction system
Merge pull request #3489 from tchaumeny/testcases_cleanup

Removed legacy code from old transaction system
Fixed negligible formatting error.
Fix a typo in 1.7 release notes
Simplified Query.build_lookup()