Issue #20812: Add a short opener to the Python 2/3 porting HOWTO.

Thanks to Nick Coghlan for the suggestion.
Issue #20813: Backport Python 2/3 HOWTO updates
null merge for issue #20812
#10197: Update get[status]output versionchanged with actual version.

This was perhaps on the border between a bug fix and a feature since
the Python3 docs did not originally say it was unix only.  However, the
functions never worked anywhere but unix, and the docs were changed to
say it was unix only well before the windows support was added.

Unfortunately, windows support was added in 3.3.4 as well as 3.4.
That leaves us in the uncomfortable position of needing the tag to say
"version changed: 3.3.4" :(
#10197: Update get[status]output versionchanged with actual version.

This was perhaps on the border between a bug fix and a feature since
the Python3 docs did not originally say it was unix only.  However, the
functions never worked anywhere but unix, and the docs were changed to
say it was unix only well before the windows support was added.

Unfortunately, windows support was added in 3.3.4 as well as 3.4.
That leaves us in the uncomfortable position of needing the tag to say
"version changed: 3.3.4" :(
whatsnew: get[status|output] works on windows.

Since 3.3.4.... :(
whatsnew: cp273 codec (#10907797)

Also updated the docs and added the aliases mentioned by the
references.
whatsnew: sunau supports 24 bit samples.
whatsnew: inet_pton/inet_ntop support windows (#7171).

Added versionchanged to docs.
whatsnew: audioop accepts any bytes-like, rejects strings.
Improve descriptions of introspection changes

Several of the introspection changes in Python 3.4 are indirect,
where inspect module changes affected pydoc, and those in turn
affected the help builtin. This update adds versionchanged
notes in the key locations, as well as more coverage in the
What's New document (in particular, a note in the porting
section regarding the expanded domain for inspect.getfullargspec).