Bump lxml from 4.6.3 to 4.9.1 (#587)

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.3 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-4.6.3...lxml-4.9.1)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -16,7 +16,7 @@ isort==4.3.21
 itsdangerous==1.1.0
 Jinja2==2.11.3
 lazy-object-proxy==1.4.3
-lxml==4.6.3
+lxml==4.9.1
 MarkupSafe==1.1.1
 mccabe==0.6.1
 nodeenv==1.4.0