Fix manhole on py3 (pt 2) (#4067)

changelog.d/4067.bugfix

@@ -0,0 +1 @@
+Manhole now works again on Python 3, instead of failing with a "couldn't match all kex parts" when connecting.

synapse/util/manhole.py

@@ -70,6 +70,8 @@ def manhole(username, password, globals):
     Returns:
         twisted.internet.protocol.Factory: A factory to pass to ``listenTCP``
     """
+    if not isinstance(password, bytes):
+        password = password.encode('ascii')
 
     checker = checkers.InMemoryUsernamePasswordDatabaseDontUse(
         **{username: password}