bpo-39259: poplib now rejects timeout = 0 (GH-17912)

poplib.POP3 and poplib.POP3_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/poplib.rst b/Doc/library/poplib.rst
index 28b42fa..2f349b3 100644
--- a/Doc/library/poplib.rst
+++ b/Doc/library/poplib.rst
@@ -47,6 +47,9 @@
       ``poplib.putline`` with arguments ``self`` and ``line``,
       where ``line`` is the bytes about to be sent to the remote host.
 
+   .. 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:: POP3_SSL(host, port=POP3_SSL_PORT, keyfile=None, certfile=None, timeout=None, context=None)
 
@@ -85,6 +88,10 @@
        :func:`ssl.create_default_context` select the system's trusted CA
        certificates for you.
 
+   .. 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.
+
 One exception is defined as an attribute of the :mod:`poplib` module:
 
 
@@ -268,4 +275,3 @@
 
 At the end of the module, there is a test section that contains a more extensive
 example of usage.
-