Use mypy to type check the runners package.
The test procedure is now annoying enough to replicate locally that
I'm going to wrap it in a script. Do this for both UNIX and Windows
environments by writing that script in Python.
Keep the GitHub workflow up to date so we now get mypy results in CI.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Martí Bolívar