Hastexo-xblock

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* [Bug fix] Make sure that any error message that is added to the
`error_msg` field of a stack, gets truncated before a stack update.
* [Testing] Include XBlock 1.5 in the test matrix, remove XBlock 1.3.
* [Testing] Include Python 3.9 in the test matrix, remove remnants of
Python 3.5 test coverage.

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* [Bug fix] Truncate the error message for `LaunchError` to fit
256 characters and thus, could be added to the `error_msg` field
of a `Stack`.

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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.