Bump lxml from 4.6.3 to 4.9.1 in /docs

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.3 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-4.6.3...lxml-4.9.1)

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

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

Author: dependabot[bot]

docs/doc-requirements.txt

@@ -24,7 +24,7 @@ joblib==1.1.0; python_version >= '3.6'
 jsmin==3.0.0
 livereload==2.6.3
 lunr[languages]==0.5.8
-lxml==4.6.3; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
+lxml==4.9.1; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
 markdown==3.3.4; python_version >= '3.6'
 markupsafe==1.1.1; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 mkdocs-exclude==1.0.2