Hastexo-xblock

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* [Enhancement] Relax version constraints in `requirements/base.txt`
so that the OpenStack Train release becomes our reference point for
OpenStack client libraries. Simultaneously, relax the version
constraints for Paramiko and Tenacity.

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* [Enhancement] Display the "check progress" button (if enabled) in
blue, to provide greater contrast to the red reset button.
* [Enhancement] Make the warning learners see when they reset a lab
more verbose. Also, add a specific warning in case the XBlock is
being displayed in a timed exam, indicating that the exam timer will
continue to run while the lab is being reset.

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* [Bug fix] Revert previous unsuccessful attempt to refactor Celery
logic.
* [Bug fix] Use the "old" Celery Task base class, which our tasks
were originally built on.

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**Do not use this release.** This contains a change breaking Celery
task invocation, and was never published on PyPI.

* [Bug fix] Refactor Celery tasks logic to define a Celery app
and register our class based tasks to that app.

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* [Bug fix] Refactor closing ssh connection in `finally` blocks.

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* [Bug fix] Add `null=True` for `key` and `password` in the Stack
model. The fix in 5.0.4 does not lead to the desired schema update
on MariaDB 10.2 (it does not change the schema at all), so rather
than using a default, allow NULL values instead.