Fixed #14170 -- Reset i18n cache when settings change
Simplify (and very very slightly speed up) the pbkdf2 implementation
Updated the set of watched files after each request.

Otherwise the kqueue-based autoreloader may not see changes to files
that weren't imported when the server started.

Thanks Bouke Haarsma for the report and Loïc Bistuer for locating the
problem.
Merge pull request #1821 from Bouke/tickets/14170

#14170 -- Reset i18n cache when settings changed
Fixed #18149 -- Changed language codes for Chinese

Language codes for Chinese are zh_Hans (Simplified) and zh_Hant (Traditional).
Added support for browsers that still send the deprecated language codes.

Thanks to Olli Wang for the report.
Merge pull request #1868 from Bouke/tickets/18419

Fixed #18149 -- Changed language codes for Chinese
Improved changelog for runserver in 1.7.
Fixed python3 breakage introduced by c0a2388a1c4ead1afaec98e4ebc953a772ca3849.