mypy: support mbedtls_dev.foo

Tell mypy to support packages without an __init__.py (PEP 420
namespace packages). Python 3.3 and (modern) Pylint support them out
of the box, but mypy needs to be told to support them.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.mypy.ini

@@ -1,3 +1,4 @@
 [mypy]
 mypy_path = scripts
+namespace_packages = True
 warn_unused_configs = True