TST: Fix RuntimeError: data type not supported on Python 2.6

On Python >=2.7, C extensions that use integer format codes with the PyArg_Parse* family of functions will raise a TypeError exception instead of triggering a DeprecationWarning (Python 2.6). The RuntimeError is raised by ndimage for unsupported array data types.
Merge pull request #787 from cgohlke/patch-2

TST: Fix RuntimeError: data type not supported on Python 2.6