Add mypy to style target (for non-driver code)

My patience with mypy is very thin, but it does catch some stuff that
other checkers don't, even outside the realm of typing. This should
at least make sure any type annotations we _do_ get are correct.

There are a lot of errors in the drivers, so limit this to just the
core code for now.

.mypy.ini

@@ -0,0 +1,5 @@
+[mypy]
+disable_error_code=import-untyped,var-annotated,import-not-found
+
+[mypy-chirp.drivers.*]
+disable_error_code=assignment

tox.ini

@@ -23,8 +23,10 @@ sitepackages = False
 deps =
     pep8
     flake8
+    mypy
 commands =
     python ./tools/cpep8.py {posargs}
+    mypy --config-file .mypy.ini chirp --exclude='/drivers/*' --exclude='chirp/share/*'
 
 [textenv:py3clean]
 commands =