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](https://github.com/lxml/lxml/compare/lxml-4.6.5...lxml-4.9.1)

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

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

requirements.txt

@@ -6,7 +6,7 @@ chardet==3.0.4
 defusedxml==0.5.0
 idna==2.8
 isodate==0.6.0
-lxml==4.6.5
+lxml==4.9.1
 pytz==2018.7
 requests==2.21.0
 requests-toolbelt==0.8.0