Bump lxml from 4.2.5 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.2.5 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-4.2.5...lxml-4.9.1)

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

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

requirements.txt

@@ -2,7 +2,7 @@ certifi==2018.8.24
 chardet==3.0.4
 feedparser==5.2.1
 idna==2.7
-lxml==4.2.5
+lxml==4.9.1
 oauthlib==2.1.0
 Pillow==5.3.0
 PySocks==1.6.8