paramiko_ssh: fix crash upon pass prompt in py3

The pass prompt expects an answer and compares a `str` to a binary buffer, thus crashing.

It's an obvious fix to help transitioning towards Python3 and hopes it does not need a specific test.
(cherry picked from commit bc44175d8d)

lib/ansible/plugins/connection/paramiko_ssh.py

@@ -291,7 +291,7 @@ class Connection(ConnectionBase):
                     chunk = chan.recv(bufsize)
                     display.debug("chunk is: %s" % chunk)
                     if not chunk:
-                        if 'unknown user' in become_output:
+                        if b'unknown user' in become_output:
                             raise AnsibleError( 'user %s does not exist' % self._play_context.become_user)
                         else:
                             break