Taught IDLE's autoident parser that "yield" is a keyword that begins a
stmt.  Along w/ the preceding change to keyword.py, making all this work
w/ a future-stmt just looks harder and harder.
diff --git a/Tools/idle/PyParse.py b/Tools/idle/PyParse.py
index a512356..c8212b2 100644
--- a/Tools/idle/PyParse.py
+++ b/Tools/idle/PyParse.py
@@ -29,6 +29,7 @@
     |   except
     |   raise
     |   import
+    |   yield
     )
     \b
 """, re.VERBOSE | re.MULTILINE).search