[deps] Update lxml requirement from ~=5.4.0 to ~=6.0.0

Updates the requirements on [lxml](https://github.com/lxml/lxml) to permit the latest version.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-5.4.0...lxml-6.0.0)

---
updated-dependencies:
- dependency-name: lxml
  dependency-version: 6.0.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-test.txt

@@ -7,7 +7,7 @@ freezegun~=1.5.2
 django-extensions
 openwisp-utils[qa,selenium,channels-test] @ https://github.com/openwisp/openwisp-utils/tarball/1.2
 pylinkvalidator
-lxml~=5.4.0
+lxml~=6.0.0
 cssselect~=1.3.0
 openwisp-monitoring @ https://github.com/openwisp/openwisp-monitoring/tarball/1.2
 django-redis~=6.0.0