[3.6] : Implement dunder iter method on mock_open bpo-32933

[TonyFlury]

…terator protocol

https://bugs.python.org/issue32933

[the-knights-who-say-ni]

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[TonyFlury]

I signed the CLA 11:10 am GMT - 4th March 2018

[Mariatta]

Thanks for the PR. Preferably the backport PR is done after the PR on master has been approved. I'm closing this.