Add defusedxml to tox deps

Until Sphinx cuts a release with [1] included.

[1] sphinx-doc/sphinx#12362

Author: hoodmane

tox.ini

@@ -15,6 +15,7 @@ deps =
     click7: click>=7.0,<8.0
     click8: click>=8.0,<9.0
     click8-async: asyncclick>=8.0,<9.0
+    defusedxml
 commands =
     python -m pytest --cov {toxinidir}/sphinx_click {posargs}
 pip_pre =