Bump lxml from 4.6.5 to 4.9.1 in /binder

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.5 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-4.6.5...lxml-4.9.1)

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

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

binder/requirements.txt

@@ -34,7 +34,7 @@ jupyterlab==1.2.21
 jupyterlab-pygments==0.1.0
 jupyterlab-server==1.0.6
 kiwisolver==1.1.0
-lxml==4.6.5
+lxml==4.9.1
 MarkupSafe==1.1.1
 matplotlib==3.1.1
 mistune==0.8.4