[issue21421] ABCs for MappingViews should declare __slots__ so subclasses aren't forced to have __dict__/__weakref__

[Raymond Hettinger]

Raymond Hettinger added the comment:

At first glance, this seems like a reasonable request.
I leave it open for comments for a while before proceeding.

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assignee:  -> rhettinger
components: +Library (Lib)
nosy: +rhettinger
type:  -> enhancement
versions: +Python 3.5

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<http://bugs.python.org/issue21421>
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