testing/py3-dt-schema: new aport

This MR adds dt-schema and it's dependency rfc3987 python packages. dt-schema is needed for running Device Tree (DT) schema checks while building the linux kernel, for example make dt_binding_check and make dtbs_check.

Also I'm not really sure what is the right way to automate the package version updates. py3-dt-schema is updated pretty much every month, the dependency is not.