[issue21762] update the import machinery to only use __spec__

[Brett Cannon]

Brett Cannon added the comment:

__package__ != __spec__.parent now triggers an ImportWarning.

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dependencies: +Raise an ImportWarning when __spec__.parent/__package__ isn't defined for a relative import

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21762>
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