devicetree: add mypy-based type checking

Extend the steps taken in tox.ini by type checking the 'devicetree'
package. This will make it easier for callers to type-check code that
uses the low level DT module's public APIs, since they can rely on
the type checker a bit more.

It will also help avoid bugs by adding some type checking for future
changes to this module.

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

scripts/dts/python-devicetree/tox.ini

@@ -5,7 +5,14 @@ envlist=py3
 deps =
     setuptools-scm
     pytest
+    mypy
 setenv =
     TOXTEMPDIR={envtmpdir}
 commands =
   python -m pytest {posargs:tests}
+  python -m mypy --config-file={toxinidir}/tox.ini --package=devicetree
+
+[mypy]
+mypy_path=src
+ignore_missing_imports=True
+