SF patch 672098: Three __contains__ implementations

Contributed by Jp Calderone.
diff --git a/Lib/lib-tk/Canvas.py b/Lib/lib-tk/Canvas.py
index ed4ba34..ae2ced1 100644
--- a/Lib/lib-tk/Canvas.py
+++ b/Lib/lib-tk/Canvas.py
@@ -39,6 +39,8 @@
         return self._keys
     def has_key(self, key):
         return key in self.keys()
+    def __contains__(self, key):
+        return key in self.keys()
     def addtag(self, tag, option='withtag'):
         self.canvas.addtag(tag, option, self.id)
     def bbox(self):