Edx-completion

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* Fix the package long description to be valid rST, check this in CI.

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* Support tracking completion of XBlocks in any "learning context", such as in
a content library, and not just in courses. To keep the code clean, this has
been done as a **breaking change** to the python API. (The API has been
simplified so that it's generally only necessary to pass in a block key /
usage key rather than block key + course key.) The REST API is unchanged.

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* Updated credentials for PyPI deployment via token.

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* Switch blocks_to_mark_complete_on_view() to return a list of XBlocks instead of a set. Many XBlocks aren't hashable;
the old implementation allowed subtle bugs under Python 2.7 but triggers an immediate error under 3.5.

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* Unpin django-rest-framework requirements. This is a potentially **breaking change** if people were
relying on this package to ensure the correct version of djangorestframework was being installed.
* Remove the AUTHORS file and references to it.

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* Fix the 403 error occurring for completion-batch API for requests coming from the iOS devices