Bump lxml from 4.8.0 to 4.9.1

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: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -32,7 +32,7 @@ importlib-metadata==4.11.1
 isodate==0.6.1
 itsdangerous==2.1.0
 Jinja2==3.0.3
-lxml==4.8.0
+lxml==4.9.1
 Markdown==3.3.6
 MarkupSafe==2.1.0
 mergedeep==1.3.4