the ready_line from the launched application is sent through
stdout which can be arbitrarily buffered in a pipe. the vtkpython
interpreter overrides sys.stdout (and others) and did not implement
flush. This patch implements flush and uses it to flush the ready_line
rather then send a bunch of data to the stream and hope that it is
enough to flush the buffer.

Change-Id: I6342e1becd459b02391e7a7998181a83c44ffff1