Bump lxml from 4.6.2 to 4.9.1 in /requirements

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.2 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-4.6.2...lxml-4.9.1)

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

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

requirements/base.txt

@@ -13,7 +13,7 @@ gevent==21.1.2
 gunicorn==20.0.4
 jsonfield2==4.0.0
 libsass==0.20.1
-lxml==4.6.2
+lxml==4.9.1
 Markdown==3.3.3
 nodeenv==1.5.0
 pytz==2021.1