Force lxml to latest version

Lower versions have a security vulnerability, see
https://github.com/lxml/lxml/security/advisories/GHSA-55x5-fj6c-h6m8

The `pyexcel-*` packages which require `lxml` don’t pin a version.

`pip-compile` will respect what’s in `requirements.txt` as long as the
dependencies are fulfilled[1] so we don’t need to add it to
`requirements.in`

---

1. https://github.com/jazzband/pip-tools#updating-requirements

requirements.txt

@@ -111,7 +111,7 @@ lml==0.1.0
     # via
     #   pyexcel
     #   pyexcel-io
-lxml==4.6.3
+lxml==4.6.5
     # via
     #   pyexcel-ezodf
     #   pyexcel-ods3