title: shapely is_valid for polygons in 3D
body: <p>I'm trying to validate some polygons that are on planes with <code>is_valid</code>, but I get  <code>Too few points in geometry component at or near point</code> for polygons where the z is not constant.</p>
  
  <p>Is there a way to validate these other polygons?</p>
  
  <p>Here's an example:</p>
  
  <pre><code>from shapely.geometry import Polygon
  
  
  poly1 = Polygon([(0,0), (1,1), (1,0)])
  print(poly1.is_valid)
  # True
  
  # z=1
  poly2 = Polygon([(0,0,1), (1,1,1), (1,0,1)])
  print(poly2.is_valid)
  # True
  
  # x=1
  poly3 = Polygon([(1,0,0), (1,1,1), (1,1,0)])
  print(poly3.is_valid) 
  # Too few points in geometry component at or near point 1 0 0
  # False
  </code></pre>