Refactor message.Answer class in a way, that it can be constructed dynamically step by step, in preparation of using streaming API.

tests/test_message.py

@@ -91,7 +91,7 @@ class QuestionTestCase(unittest.TestCase):
 class AnswerTestCase(unittest.TestCase):
     def test_answer_with_header(self) -> None:
         with self.assertRaises(MessageError):
-            Answer(f"{Answer.txt_header}\nno")
+            str(Answer(f"{Answer.txt_header}\nno"))
 
     def test_answer_with_legal_header(self) -> None:
         answer = Answer(f"This is a line contaning '{Answer.txt_header}'\nIt is what it is.")