Patch by Matej Cepl.

Reverts some changes of d13fdd97cc8e.

`versionchanged` tags about 2.7 are useless in 3.x branches.

It doesn't get called on 'null', 'true', 'false' since f686aced02a3.

 Issue #14426: Correct the Date format in Expires attribute of Set-Cookie. Patch by Federico Reghenzani and Müte Invert

Closes #14842.

The behaviour of the parse_constant callback changed insofar that 'null',
'true', 'false' don't trigger its call anymore.

Patch by Serhiy Storchaka

undecoded UTF-8 bytes as a string when accessing the Tk clipboard.
Modify clipboad_get() to first request type UTF8_STRING when no
specific type is requested in an X11 windowing environment, falling
back to the current default type STRING if that fails.
Original patch by Thomas Kluyver.

 G: changed Misc/NEWS

This patch also clarifies the definition of Naive and Aware.

Original patch by Greg Weller, I modified the first hunk
somewhat to make the exposition even clearer (I hope).

Patch by Hynek.