Also adds the TOC entry for headerregistry.

When I made the checkin of the provisional email policy, I knew that
Address and Group needed to be made accessible from somewhere.  The more
I looked at it, though, the more it became clear that since this is a
provisional API anyway, there's no good reason to hide headerregistry as
a private API.  It was designed to ultimately be part of the public API,
and so it should be part of the provisional API.

This patch fully documents the headerregistry API, and deletes the
abbreviated version of those docs I had added to the provisional policy
docs.

Close #14857: fix regression in references to PEP 3135 implicit __class__ closure variable. Reopens issue #12370, but also updates unittest.mock to workaround that issue

Tweak importlib._bootstrap to avoid zero-argument super so I can work on issue #14857 without breaking imports

Issue #10365: File open dialog now works instead of crashing
even when parent window is closed. Patch by Roger Serwy.

even when parent window is closed. Patch by Roger Serwy.

Patch by Roger Serwy.

Patch by Roger Serwy.

smtpd now handles EHLO and has infrastructure for extended smtp command mode.
The SIZE extension is also implemented.  In order to support parameters on
MAIL FROM, the RFC 5322 parser from the email package is used to parse the
address "token".

Logging subclasses things and overrides __init__, so it was necessary to
update those __init__ functions in the logging tests to make the logging tests
pass.

The original suggestion and patch were by Alberto Trevino.  Juhana Jauhiainen
added the --size argument and SIZE parameter support.  Michele Orrù improved
the patch and added more tests.  Dan Boswell conditionalized various bits of
code on whether or not we are in HELO or EHLO mode, as well as some other
improvements and tests.  I finalized the patch and added the address parsing.

Although '<>' is invalid according to RFC 5322, SMTP uses it for various
things, and it sometimes ends up in email headers.  This patch changes
get_angle_addr to recognize it and just register a Defect instead of raising a
parsing error.

Issue #14814: Add a basic ipaddress tutorial (thanks to Sandro Tosi for the initial conversion from Peter Moody's wiki version)

Issue #14814: In the spirit of TOOWTDI, ditch the redundant version parameter to the factory functions by using the appropriate direct class references instead

Without this function people would be tempted to use the other date functions
in email.utils to compute an aware localtime, and those functions are not as
good for that purpose as this code.  The code is Alexander Belopolsy's from
his proposed patch for issue 9527, with a fix (and additional tests) by Brian
K. Jones.

This is a danger of focusing on unit tests: sometimes you forget
to do the integration tests.

When the new policies are used (and only when the new policies are explicitly
used) headers turn into objects that have attributes based on their parsed
values, and can be set using objects that encapsulate the values, as well as
set directly from unicode strings.  The folding algorithm then takes care of
encoding unicode where needed, and folding according to the highest level
syntactic objects.

With this patch only date and time headers are parsed as anything other than
unstructured, but that is all the helper methods in the existing API handle.
I do plan to add more parsers, and complete the set specified in the RFC
before the package becomes stable.