Add stream parameter to getpass

[Carreau]

'cause getpass.getpass takes stream as an argument.

[takluyver]

======================================================================
ERROR: Tests that kernel getpass accept the stream parameter
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/ipython/ipykernel/ipykernel/inprocess/tests/test_kernel.py", line 74, in test_getpass_stream
    kernel.getpass(stream='non empty')
  File "/home/travis/build/ipython/ipykernel/ipykernel/kernelbase.py", line 668, in getpass
    "getpass was called, but this frontend does not support input requests."
IPython.core.error.StdinNotImplementedError: getpass was called, but this frontend does not support input requests.

[Carreau]

Travis should be happy. GitHub show the commits in the wrong order.