Bump lxml from 4.6.3 to 4.9.1 in /requirements

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/base.txt

@@ -24,4 +24,4 @@ mailchimp3==3.0.17
 django-ical==1.8.3
 TwitterApi==2.8.1
 facebook-sdk==3.1.0
-lxml==4.9.0
+lxml==4.9.1