Bump lxml from 5.1.0 to 5.2.1 in /requirements in the pypi group

Bumps the pypi group in /requirements with 1 update: [lxml](https://github.com/lxml/lxml).


Updates `lxml` from 5.1.0 to 5.2.1
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-5.1.0...lxml-5.2.1)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: pypi
...

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

requirements/tests.txt

@@ -7,7 +7,7 @@ csscompressor==0.9.5
 django-sekizai==4.1.0
 flake8==7.0.0
 html5lib==1.1
-lxml==5.1.0
+lxml==5.2.1
 rcssmin==1.1.2
 rjsmin==1.2.2
 slimit==0.8.1