[2.7] bpo-1967: proxy dictviews in dictproxy

[graingert]

https://bugs.python.org/issue1967

[the-knights-who-say-ni]

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[jdemeyer]

The title should start with [2.7] to make it clear that this is for 2.7 only.

[vstinner]

People survived without these 3 methods. It's too late to "complete" Python 2.7: it will no longer be supported in 6 months. I close this PR.