Bump lxml from 4.5.2 to 4.9.1

Bumps [lxml](https://github.com/lxml/lxml) from 4.5.2 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-4.5.2...lxml-4.9.1)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 -e .
 -e git+https://github.com/openedx/xblock-utils.git@2.1.1#egg=xblock-utils==2.1.1
 edx-i18n-tools==0.5.3
-lxml==4.5.2
+lxml==4.9.1
 mako==1.1.3
 parsel==1.6.0
 transifex-client==0.13.11