Fixes the use of the keys() method

Signed-off-by: Voldivh <eloyabmfcv@gmail.com>

Author: Voldivh

rosidl_generator_py/resource/_msg.py.em

@@ -372,7 +372,7 @@ if isinstance(type_, AbstractNestedType):
         typename.pop()
         typename.append(self.__class__.__name__)
         args = []
-        for s, t in zip(self.get_fields_and_field_types().keys, self.SLOT_TYPES):
+        for s, t in zip(self.get_fields_and_field_types().keys(), self.SLOT_TYPES):
             field = getattr(self, s)
             fieldstr = repr(field)
             # We use Python array type for fields that can be directly stored