[issue21145] Add the @cached_property decorator

[Carl Meyer]

Carl Meyer added the comment:

Attaching a patch with the simplest version of cached_property (tehnique is not original, similar code is found in Django, Bottle, Flask, the cached_property lib, and many other places).

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components: +Library (Lib)
keywords: +patch
versions: +Python 3.7 -Python 3.6
Added file: http://bugs.python.org/file45431/cached_property.diff

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