Bump lxml from 4.9.1 to 6.1.0 in /docs (#1734)

Bumps [lxml](https://github.com/lxml/lxml) from 4.9.1 to 6.1.0.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.9.1...lxml-6.1.0)

---
updated-dependencies:
- dependency-name: lxml
  dependency-version: 6.1.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Michael Hank <mhank@sas.upenn.edu>

Author: dependabot[bot]

docs/requirements.txt

@@ -10,7 +10,7 @@ exhale==0.2.4
 idna==3.7
 imagesize==1.1.0
 Jinja2==3.1.6
-lxml==4.9.1
+lxml==6.1.0
 MarkupSafe==2.1.3
 packaging==19.2
 Pygments==2.15.0