[issue9523] Improve dbm modules

[Ray.Allen]

Ray.Allen <ysj.ray at gmail.com> added the comment:

Thanks!

Here is my updated patch:
1, Now the dbm view objects are the same as dict view objects, which are in conformity with collections.KeysView, ValuesView and ItemsView.
2, I register all these abcs explicitly because these abcs have not __subclasshook__() method so they can't check api conformance(at lease exist) through isinstance(). I could not make sure api conformance except testing each method I find in abc explicitly. And my test_abc() is just to test the registering.
3, Other updates which are from comments I wrote newly on codereview: http://codereview.appspot.com/4185044/

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Added file: http://bugs.python.org/file20771/issue_9523.diff

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