python-devicetree: tox: fix mypy

Recent versions of mypy have learned that the yaml module has type
stubs and the tool is now erroring out when it discovers we import
yaml since the stubs are not involved.

This is breaking CI on unrelated patches; fix it following the
instructions here:

https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>