Patch by Priya Pappachan, based on suggestions from Terry Reedy
and myself.

Patch by Claudiu Popa.

More tests of the concrete policy methods would probably be
a good idea, but this is a start, and it brings line coverage
up to 100% for the policy module.

Patch by Milan Oberkirch.

Also fixed the versionchanged/deprecation tags in the docs.

I decided to leave the (slightly expanded) Other Improvements
entry as well, since someone looking for command line behavior
changes isn't going to look in the 'sys' module section.

This fifth tuple element is unsupported in an old hg version still used on
one buildbot, and it is not really necessary (only used in touch --help).

Also updated the base64 module title and introduction to adjust for
the fact that these new encodings are included.

Also adjusted the wording about the base64 alphabets (see
issue #20837).

Also clarify the documentation of this feature.