Background: see http://trac.gispython.org/lab/ticket/168

  >>> from shapely.geometry import Polygon
  >>> coords = ((0.0, 0.0, 0.0), (0.0, 1.0, 0.0), (1.0, 1.0, 0.0), (1.0, 0.0, 0.0))
  >>> polygon = Polygon(coords)
  >>> polygon._ndim
  3
  >>> gi = polygon.__geo_interface__
  >>> gi['coordinates']
  (((0.0, 0.0, 0.0), (0.0, 1.0, 0.0), (1.0, 1.0, 0.0), (1.0, 0.0, 0.0), (0.0, 0.0, 0.0)),)

  >>> e = polygon.exterior
  >>> e._ndim
  3
  >>> gi = e.__geo_interface__
  >>> gi['coordinates']
  ((0.0, 0.0, 0.0), (0.0, 1.0, 0.0), (1.0, 1.0, 0.0), (1.0, 0.0, 0.0), (0.0, 0.0, 0.0))