Improve DictMixin.

Replaced docstring with comments.  Prevents subclass contamination.
Added the missing __cmp__() method and a test for __cmp__().
Used try/except style in preference to has_key() followed by a look-up.
Used iteritem() where possible to save creating a long key list and
   to save redundant lookups.
Expanded .update() to look for the most helpful methods first and gradually
   work down to a mininum expected interface.
Expanded documentation to be more clear on how to use the class.

Lib/test/test_userdict.py

@@ -210,9 +210,8 @@ else:
 s.update({10: 'ten', 20:'twenty'})                          # update
 verify(s[10]=='ten' and s[20]=='twenty')
 
-
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+verify(s == {10: 'ten', 20:'twenty'})                       # cmp
+t = SeqDict()
+t[20] = 'twenty'
+t[10] = 'ten'
+verify(s == t)