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

@@ -52,7 +52,7 @@ inotify-simple==1.3.5; python_version >= '2.7' and python_version not in '3.0, 3
 inotifyrecursive==0.3.5
 joblib==1.0.1; python_version >= '3.6'
 langdetect==1.0.9
-lxml==4.6.3; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
+lxml==4.9.1; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'
 msgpack==1.0.2
 numpy==1.20.3
 ocrmypdf==12.3.2