--- a/paramiko/agent.py
+++ b/paramiko/agent.py
@@ -415,9 +415,6 @@ class AgentKey(PKey):
     def asbytes(self):
         return self.blob
 
-    def __str__(self):
-        return self.asbytes()
-
     def get_name(self):
         return self.name
 
--- a/paramiko/message.py
+++ b/paramiko/message.py
@@ -53,10 +53,7 @@ class Message(object):
         else:
             self.packet = BytesIO()
 
-    def __str__(self):
-        """
-        Return the byte stream content of this message, as a string/bytes obj.
-        """
+    def __bytes__(self):
         return self.asbytes()
 
     def __repr__(self):
--- a/paramiko/pkey.py
+++ b/paramiko/pkey.py
@@ -118,7 +119,7 @@ class PKey(object):
         """
         return bytes()
 
-    def __str__(self):
+    def __bytes__(self):
         return self.asbytes()
 
     # noinspection PyUnresolvedReferences
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -48,3 +48,10 @@ class AgentTests(unittest.TestCase):
             kwargs=dict(algorithm="rsa-sha2-512"),
             expectation=SSH_AGENT_RSA_SHA2_512,
         )
+
+    def test_agent_key_str_kinda_fixed(self):
+        # Tests for a missed spot in Python 3 upgrades: AgentKey.__str__ was
+        # returning bytes, as if under Python 2. When bug present, this
+        # explodes with "__str__ returned non-string".
+        key = AgentKey(ChaosAgent(), b("secret!!!"))
+        assert str(key) == repr(key)
--- a/tests/test_message.py
+++ b/tests/test_message.py
@@ -105,3 +105,9 @@ class MessageTest(unittest.TestCase):
         self.assertEqual(msg.get_adaptive_int(), 5)
         self.assertEqual(msg.get_so_far(), self.__d[:4])
         self.assertEqual(msg.get_remainder(), self.__d[4:])
+
+    def test_bytes_str_and_repr(self):
+        msg = Message(self.__d)
+        assert str(msg) == f"paramiko.Message({self.__d!r})"
+        assert repr(msg) == str(msg)
+        assert bytes(msg) == msg.asbytes() == self.__d