Fix test failure on Python 2. Ref #3314.

_pytest/capture.py

@@ -560,7 +560,7 @@ class SysCaptureBinary(SysCapture):
         return res
 
 
-class DontReadFromInput(object):
+class DontReadFromInput(six.Iterator):
     """Temporary stub class.  Ideally when stdin is accessed, the
     capturing should be turned off, with possibly all data captured
     so far sent to the screen.  This should be configurable, though,