Bump lxml from 4.5.0 to 4.9.1 in /requirements

Bumps [lxml](https://github.com/lxml/lxml) from 4.5.0 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-4.5.0...lxml-4.9.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/base.txt

@@ -5,7 +5,7 @@ Flask-Caching==1.8.0
 redis==3.5.2
 Flask-DotEnv==0.1.1
 python-dotenv==0.10.1
-lxml==4.5.0
+lxml==4.9.1
 requests==2.23.0
 pandas==1.0.3
 tweepy==3.8.0