SF bug #719367, string exceptions are deprecated

Remove references to string based exceptions in the doc.

Doc/lib/libexcs.tex

@@ -4,21 +4,26 @@
 \modulesynopsis{Standard exception classes.}
 
 
-Exceptions can be class objects or string objects.  Though most
-exceptions have been string objects in past versions of Python, in
-Python 1.5 and newer versions, all standard exceptions have been
-converted to class objects, and users are encouraged to do the same.
+Exceptions should be class objects.  
 The exceptions are defined in the module \module{exceptions}.  This
 module never needs to be imported explicitly: the exceptions are
 provided in the built-in namespace as well as the \module{exceptions}
 module.
 
+\begin{notice}
+In past versions of Python string exceptions were supported.  In
+Python 1.5 and newer versions, all standard exceptions have been
+converted to class objects and users are encouraged to do the same.
+String exceptions will raise a \code{PendingDeprecationWarning}.
+In future versions, support for string exceptions will be removed.
+
 Two distinct string objects with the same value are considered different
 exceptions.  This is done to force programmers to use exception names
 rather than their string value when specifying exception handlers.
 The string value of all built-in exceptions is their name, but this is
 not a requirement for user-defined exceptions or exceptions defined by
 library modules.
+\end{notice}
 
 For class exceptions, in a \keyword{try}\stindex{try} statement with
 an \keyword{except}\stindex{except} clause that mentions a particular