gh-55454: Add IMAP4 IDLE support to imaplib

gh-55454: Add IMAP4 IDLE support to imaplib

This extends imaplib with support for the rfc2177 IMAP IDLE command,
as requested in #55454.  It allows events to be pushed to a client as
they occur, rather than having to continually poll for mailbox changes.

The interface is a new idle() method, which returns an iterable context
manager.  Entering the context starts IDLE mode, during which events
(untagged responses) can be retrieved using the iteration protocol.
Exiting the context sends DONE to the server, ending IDLE mode.

An optional time limit for the IDLE session is supported, for use with
servers that impose an inactivity timeout.

The context manager also offers a burst() method, designed for programs
wishing to process events in batch rather than one at a time.

Notable differences from other implementations:

- It's an extension to imaplib, rather than a replacement.
- It doesn't introduce additional threads.
- It doesn't impose new requirements on the use of imaplib's existing methods.
- It passes the unit tests in CPython's test/test_imaplib.py module
  (and adds new ones).
- It works on Windows, Linux, and other unix-like systems.
- It makes IDLE available on all of imaplib's client variants
  (including IMAP4_stream).
- The interface is pythonic and easy to use.

Caveats:

- Due to a Windows limitation, the special case of IMAP4_stream running
  on Windows lacks a duration/timeout feature. (This is the stdin/stdout
  pipe connection variant; timeouts work fine for socket-based
  connections, even on Windows.) I have documented it where appropriate.

- The file-like imaplib instance attributes are changed from buffered to
  unbuffered mode. This could potentially break any client code that
  uses those objects directly without expecting partial reads/writes.
  However, these attributes are undocumented. As such, I think (and
  PEP 8 confirms) that they are fair game for changes.
  https://peps.python.org/pep-0008/#public-and-internal-interfaces

Usage examples:

https://github.com/python/cpython/issues/55454#issuecomment-2227543041

Original discussion:

https://discuss.python.org/t/gauging-interest-in-my-imap4-idle-implementation-for-imaplib/59272

Earlier requests and suggestions:

https://github.com/python/cpython/issues/55454

https://mail.python.org/archives/list/python-ideas@python.org/thread/C4TVEYL5IBESQQPPS5GBR7WFBXCLQMZ2/