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.