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.
diff --git a/Lib/decimal.py b/Lib/decimal.py
index 875e38a..2f989a8 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -2248,7 +2248,7 @@
         s.append('traps=[' + ', '.join([t.__name__ for t, v in self.traps.items() if v]) + ']')
         return ', '.join(s) + ')'
 
-    def __context__(self):
+    def context_manager(self):
         return WithStatementContext(self.copy())
 
     def clear_flags(self):