Log Message:
py-xmlschema: updated to 1.6.4

v1.6.4
* Add testing config for Python 3.10 (Tox and CI)
* Fix internal _PurePath class with Python 3.10
* Remove redundant xmlns="" declaration when encoding with lxml

v1.6.3
* Refactor normalize_url() using pathlib.PurePath
* Support UNC paths
* Fix API docs