Fix SF bug #763023, difflib.py: ratio() zero division not caught

Backport candidate
diff --git a/Misc/NEWS b/Misc/NEWS
index 76ec673..d3761f9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -24,6 +24,9 @@
 Library
 -------
 
+- SF bug 763023: fix uncaught ZeroDivisionError in difflib ratio methods
+  when there are no lines.
+
 Tools/Demos
 -----------