Get rid of __context__, per the latest changes to PEP 343 and python-dev

discussion.
There are two places of documentation that still mention __context__:
Doc/lib/libstdtypes.tex -- I wasn't quite sure how to rewrite that without
spending a whole lot of time thinking about it; and whatsnew, which Andrew
usually likes to change himself.

Lib/dummy_thread.py

@@ -118,9 +118,6 @@ class LockType(object):
     def __exit__(self, typ, val, tb):
         self.release()
 
-    def __context__(self):
-        return self
-
     def release(self):
         """Release the dummy lock."""
         # XXX Perhaps shouldn't actually bother to test?  Could lead