Bump lxml from 4.6.3 to 4.9.1

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>

requirements.txt

@@ -57,7 +57,7 @@ liac-arff==2.4.0
 llvmlite==0.30.0
 lockfile==0.12.2
 louis==3.5.0
-lxml==4.6.3
+lxml==4.9.1
 macaroonbakery==1.1.3
 Mako==1.0.7
 MarkupSafe==1.1.1