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](https://github.com/lxml/lxml/compare/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

@@ -5,7 +5,7 @@ dnspython       ==2.0.0
 feedparser      ==6.0.2
 html5lib        ==1.0.1
 isodate         ==0.6.0
-lxml            ==4.6.3
+lxml            ==4.9.1
 netifaces       ==0.10.9
 PySocks         ==1.7.1
 python-dateutil ==2.8.1