Edx-completion

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* Fix AttributeError raised by `vertical_is_complete`.
* by ensuring `get_completable_children` doesn't return null

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* Updated the children lookup for `vertical_is_complete` to utilize the XBlockCompletion model. There are
three completion modes to consider: EXCLUDED, AGGREGATOR, COMPLETABLE.

* This method will now ignore any block with XBlockCompletion.EXCLUDED.
* This method will now recurse down any child of a vertical if that child has XBlockCompletion.AGGREGATOR.
* This method will consider all children blocks with XBlockCompletion.COMPLETABLE as candidates to
determine if the vertical is complete.

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* Adding recursive lookup for children of a vertical to the `vertical_is_complete` method in services.py.

* This was added because verticals containing children that had their own children were not being properly marked
as complete. Since the vertical was only looking one layer deep, it was possible to have children lower in the tree
incomplete, but the vertical would still be marked as complete. Now it looks at all leaves under the vertical.

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* Remove unnecessary constraint for edx-drf-extensions<3.0.0

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* Upgrades packages, drops support for Python 2.

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