build(deps-dev): bump pytest-xdist in /src/karmen_backend

Bumps [pytest-xdist](https://github.com/pytest-dev/pytest-xdist) from 1.34.0 to 2.1.0.
- [Release notes](https://github.com/pytest-dev/pytest-xdist/releases)
- [Changelog](https://github.com/pytest-dev/pytest-xdist/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-xdist@v1.34.0...v2.1.0)

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