mypy: Add a place to put type stubs.

This will allow us to begin to add our own stubs for external
libraries.  Writing stubs can be surprisingly little work to do, and
can have high leverage in keeping our type annotations high-quality.

mypy.ini

@@ -2,6 +2,7 @@
 # Logistics of what code to check and how to handle the data.
 scripts_are_modules = True
 show_traceback = True
+mypy_path = stubs/
 cache_dir = var/mypy-cache
 
 # Options to make the checking stricter.

tools/run-mypy

@@ -16,6 +16,7 @@ sys.path.append(os.path.dirname(TOOLS_DIR))
 from lib.test_script import get_provisioning_status
 
 exclude = """
+stubs/
 """.split()
 
 parser = argparse.ArgumentParser(description="Run mypy on files tracked by git.")