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

@@ -53,7 +53,7 @@ jupyter-server-mathjax==0.2.3
 jupyter-sphinx==0.3.2
 jupyterlab-widgets==1.0.0
 livereload==2.6.3
-lxml==4.8.0
+lxml==4.9.1
 markdown-it-py==1.1.0
 MarkupSafe==2.1.1
 matplotlib-inline==0.1.2