Fix im_self access in Python 3.

skimage/transform/_geometric.py

@@ -5,6 +5,8 @@ from skimage.util import img_as_float
 from ._warps_cy import _warp_fast
 
 from skimage._shared.utils import get_bound_method_class
+from skimage._shared import six
+
 
 class GeometricTransform(object):
     """Perform geometric transformations on a set of coordinates.
@@ -1012,7 +1014,7 @@ def warp(image, inverse_map=None, map_args={}, output_shape=None, order=1,
         elif hasattr(inverse_map, '__name__') \
              and inverse_map.__name__ == 'inverse' \
              and get_bound_method_class(inverse_map) in HOMOGRAPHY_TRANSFORMS:
-            matrix = np.linalg.inv(inverse_map.im_self._matrix)
+            matrix = np.linalg.inv(six.get_method_self(inverse_map)._matrix)
         if matrix is not None:
             # transform all bands
             dims = []