Bump lxml from 4.6.5 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.5 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-4.6.5...lxml-4.9.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -11,7 +11,7 @@ requests==2.26.0
 tqdm==4.46.1
 joblib==0.16.0
 sklearn_crfsuite==0.3.6
-lxml==4.6.5
+lxml==4.9.1
 pytest==6.0.2
 pytest-cov==2.10.1
 pytest-mock==3.3.1