Bump lxml from 4.3.4 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.3.4 to 4.9.1.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](https://github.com/lxml/lxml/compare/lxml-4.3.4...lxml-4.9.1)

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

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

requirements.txt

@@ -78,7 +78,7 @@ jsonfield==2.0.2
 kombu==4.4.0
 kubernetes==9.0.0
 lockfile==0.12.2
-lxml==4.3.4
+lxml==4.9.1
 Mako==1.0.12
 Markdown==2.6.11
 MarkupSafe==1.1.1