[issue21991] The new email API should use MappingProxyType instead of returning new dicts.

[R. David Murray]

R. David Murray added the comment:

Personally I would test that the returned object is read only, rather than checking for MappingProxyType explicitly, but you can argue either way as being better :)

As for other occurrences, I must have been either misremembering, or I refactored the other occurrences out of existence earlier.

Yes I will apply this.

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stage: patch review -> commit review

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<http://bugs.python.org/issue21991>
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