Issue23806 - Document the no_proxy environment variable in Python 3 docs.

Doc/library/urllib.request.rst

@@ -269,6 +269,11 @@ The following classes are provided:
 
    To disable autodetected proxy pass an empty dictionary.
 
+   The :envvar:`no_proxy` environment variable can be used to specify hosts
+   which shouldn't be reached via proxy; if set, it should be a comma-separated
+   list of hostname suffixes, optionally with ``:port`` appended, for example
+   ``cern.ch,ncsa.uiuc.edu,some.host:8080``.
+
 
 .. class:: HTTPPasswordMgr()