Bump lxml from 4.5.1 to 4.9.1 in /requirements

Bumps [lxml](https://github.com/lxml/lxml) from 4.5.1 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-4.5.1...lxml-4.9.1)

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

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

requirements/prod.txt

@@ -5,7 +5,7 @@ click==7.1.2
 elasticsearch==7.8.0
 idna==2.10
 langdetect==1.0.8
-lxml==4.5.1
+lxml==4.9.1
 PySocks==1.7.1
 PyYAML==5.3.1
 requests==2.24.0