Add python runtime specifier requirement

Signed-off-by: Ivan Kanakarakis <[email protected]>

Author: c00kiemon5ter

setup.cfg

@@ -44,6 +44,7 @@ scripts =
     tools/mdexport.py
     tools/merge_metadata.py
     tools/parse_xsd2.py
+python_requires = >=3.6, <4
 install_requires =
     cryptography >= 1.4
     defusedxml