Bump lxml from 4.8.0 to 4.9.1 in /backend

Bumps [lxml](https://github.com/lxml/lxml) from 4.8.0 to 4.9.1.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.8.0...lxml-4.9.1)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: indirect
...

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Author: dependabot[bot]