specify minimum version needed for xmlschema

Sandbox mode was adding in 1.2.0 of python-xmlschema and refined in
1.2.1. Its use was added in 3b70772.

Author: JanZerebecki

setup.cfg

@@ -54,7 +54,7 @@ install_requires =
     requests >= 1.0.0
     six
     importlib_resources
-    xmlschema
+    xmlschema >= 1.2.1
 
 
 [options.packages.find]