Bump lxml from 4.6.2 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.2 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-4.6.2...lxml-4.9.1)

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

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

requirements.txt

@@ -111,7 +111,7 @@ langdetect==1.0.7
 lazy-object-proxy==1.4.3
 lightgbm==2.3.1
 llvmlite==0.31.0
-lxml==4.6.2
+lxml==4.9.1
 lz4==3.0.2
 Mako==1.1.1
 Markdown==3.1