bpo-39259: nntplib.NNTP/NNTP_SSL now reject timeout = 0 (GH-17936)

nntplib.NNTP and nntplib.NNTP_SSL now raise a ValueError
if the given timeout for their constructor is zero to
prevent the creation of a non-blocking socket.
diff --git a/Doc/library/nntplib.rst b/Doc/library/nntplib.rst
index e8480b5..7697365 100644
--- a/Doc/library/nntplib.rst
+++ b/Doc/library/nntplib.rst
@@ -93,6 +93,10 @@
    .. versionchanged:: 3.3
       Support for the :keyword:`with` statement was added.
 
+   .. versionchanged:: 3.9
+      If the *timeout* parameter is set to be zero, it will raise a
+      :class:`ValueError` to prevent the creation of a non-blocking socket.
+
 .. class:: NNTP_SSL(host, port=563, user=None, password=None, ssl_context=None, readermode=None, usenetrc=False, [timeout])
 
    Return a new :class:`NNTP_SSL` object, representing an encrypted
@@ -122,6 +126,10 @@
       :attr:`ssl.SSLContext.check_hostname` and *Server Name Indication* (see
       :data:`ssl.HAS_SNI`).
 
+   .. versionchanged:: 3.9
+      If the *timeout* parameter is set to be zero, it will raise a
+      :class:`ValueError` to prevent the creation of a non-blocking socket.
+
 .. exception:: NNTPError
 
    Derived from the standard exception :exc:`Exception`, this is the base