message: Add type hints to schema_test.py
diff: @@ -5,10 +5,10 @@ from onnx import defs, AttributeProto
  class TestSchema(unittest.TestCase):
  - def test_get_schema(self):
  + def test_get_schema(self): # type: () -> None
  defs.get_schema("Relu")
  - def test_typecheck(self):
  + def test_typecheck(self): # type: () -> None
  defs.get_schema("Conv")
  def test_attr_default_value(self):
  