Bump lxml from 6.0.4 to 6.1.0 in /doc/source

Bumps [lxml](https://github.com/lxml/lxml) from 6.0.4 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-6.0.4...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>

Author: dependabot[bot]

doc/source/requirements.txt

@@ -1,4 +1,4 @@
-lxml==6.0.4
+lxml==6.1.0
 importlib_metadata;python_version < '3.8'
 packaging
 Sphinx>=3