Labgrid

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FIXME

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- Some client commands return 0 even if the command failed.

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- In `USBVideoDriver`, use the ``playbin3`` element instead of ``playbin`` to
fix decoding via VA-API for certain webcams on AMD graphic cards.
- Let the `SSHDriver` redirect ``/dev/null`` to stdin on ``run()`` to prevent
unexpected consumption of stdin of the remotely started process.
- Cover more failure scenarios in the exporter and coordinator systemd
services, fix the service startup order, do not buffer journal logs.

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- Fix readthedocs build by specifying Python version and OS.
- Fix several incompatibilities with doc sphinxcontrib-* dependencies having
dropped their explicit Sphinx dependencies, which prevented generation of
labgrid's docs.

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- Fix dockerfiles syntax error that became fatal in a recent docker release.
- Fix ShellDriver's xmodem functionality.
- Pin pylint to prevent incompatibility with pinned pytest-pylint.
- Fix ``labgrid-client console --loop`` on disappearing serial ports (such as
on-board FTDIs).

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- Update to PyYAML 6.0.1 to prevent install errors with Cython>=3.0, see:
https://github.com/yaml/pyyaml/issues/601
https://github.com/yaml/pyyaml/pull/726#issuecomment-1640397938