[issue17621] Create a lazy import loader mixin

[Brett Cannon]

Brett Cannon added the comment:

Another update to trigger loading on attribute deletions as well as detecting when an import swapped the module in sys.modules, raising ValueError if detected since it won't have the affect that's expected (could be convinced to make that ImportError instead).

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Added file: http://bugs.python.org/file34591/lazy_loader.diff

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