fix check for passwordless OpenSSH (fixes #2266)

pexpect takes a regular expression, so we want '[Pp]assword:', not
'[Ppassword]:'.

IPython/external/ssh/tunnel.py

@@ -97,7 +97,7 b' def _try_passwordless_openssh(server, keyfile):'
     p = pexpect.spawn(cmd)
     while True:
         try:
-            p.expect('[Ppassword]:', timeout=.1)
+            p.expect('[Pp]assword:', timeout=.1)
         except pexpect.TIMEOUT:
             continue
         except pexpect.EOF: