Bump lxml from 3.7.3 to 4.9.1 in /openstack_requirements/tests/files

Bumps [lxml](https://github.com/lxml/lxml) from 3.7.3 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/commits/lxml-4.9.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

openstack_requirements/tests/files/upper-constraints.txt

@@ -21,7 +21,7 @@ osprofiler===1.6.0
 bandit===1.4.0
 tabulate===0.7.7
 python-ironic-inspector-client===1.11.0
-lxml===3.7.3
+lxml===4.9.1
 python-kingbirdclient===0.1.0
 pytest===3.0.6
 python-etcd===0.4.5