gh-104773: PEP 594: Remove the nntplib module (#104894)

* socket_helper.transient_internet() no longer imports nntplib to
  catch nntplib.NNTPTemporaryError.
* ssltests.py no longer runs test_nntplib.
* "make quicktest" no longer runs test_nntplib.
* WASM: remove nntplib from OMIT_NETWORKING_FILES.
* Remove mentions to nntplib in the email documentation.

Author: vstinner

Doc/library/email.generator.rst

@@ -10,8 +10,8 @@
 
 One of the most common tasks is to generate the flat (serialized) version of
 the email message represented by a message object structure.  You will need to
-do this if you want to send your message via :meth:`smtplib.SMTP.sendmail` or
-the :mod:`nntplib` module, or print the message on the console.  Taking a
+do this if you want to send your message via :meth:`smtplib.SMTP.sendmail`,
+or print the message on the console.  Taking a
 message object structure and producing a serialized representation is the job
 of the generator classes.
 

Doc/library/email.rst

@@ -15,7 +15,7 @@
 The :mod:`email` package is a library for managing email messages.  It is
 specifically *not* designed to do any sending of email messages to SMTP
 (:rfc:`2821`), NNTP, or other servers; those are functions of modules such as
-:mod:`smtplib` and :mod:`nntplib`.  The :mod:`email` package attempts to be as
+:mod:`smtplib`.  The :mod:`email` package attempts to be as
 RFC-compliant as possible, supporting :rfc:`5322` and :rfc:`6532`, as well as
 such MIME-related RFCs as :rfc:`2045`, :rfc:`2046`, :rfc:`2047`, :rfc:`2183`,
 and :rfc:`2231`.
@@ -141,9 +141,6 @@ Legacy API:
    Module :mod:`imaplib`
       IMAP (Internet Message Access Protocol) client
 
-   Module :mod:`nntplib`
-      NNTP (Net News Transport Protocol) client
-
    Module :mod:`mailbox`
       Tools for creating, reading, and managing collections of messages on disk
       using a variety standard formats.