Bump lxml from 4.6.3 to 4.6.4

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.3 to 4.6.4.
- [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-4.6.3...lxml-4.6.4)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

requirements.txt

@@ -11,7 +11,7 @@ drf-writable-nested==0.6.3
 bleach==4.1.0
 bleach-allowlist==1.0.3
 gunicorn==20.1.0
-lxml==4.6.3
+lxml==4.6.4
 Markdown==3.3.6
 Pillow==8.4.0
 psycopg2-binary==2.9.1