Bump lxml from 4.5.0 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.5.0 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-4.5.0...lxml-4.9.1)

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

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

requirements.txt

@@ -22,7 +22,7 @@ jupyter-core==4.6.1
 jupyter==1.0.0
 jupyterlab-server==0.2.0
 jupyterlab==0.35.4
-lxml==4.5.0
+lxml==4.9.1
 mistune==0.8.4
 nbconvert==6.0.0a0
 nbformat==5.0.0