Hastexo-xblock

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* [Bug fix] Make XBlock exports (from Studio or its REST API)
deterministic and predictable.
* [Enhancement] Add tests for the new export/import logic.
* [Bug fix] Restrict stack names to ASCII characters and digits.

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* [Bug fix] Fix RDP connectivity check for IPv6 stacks.

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* [Enhancement] Speed up progress checks by reducing the sleep time when
waiting for a remote execution of a command to finish.

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* [Bug fix] Add constraints to `dogpile.cache` and `cliff`, so that our
OpenStack client libraries will not have dependency conflicts.

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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.