double tolerance for test_png.py/pngsuite on Windows

lib/matplotlib/tests/test_png.py

@@ -17,7 +17,7 @@
 
 
 @image_comparison(baseline_images=['pngsuite'], extensions=['png'],
-                  tol=0.01 if on_win else 0)
+                  tol=0.02 if on_win else 0)
 def test_pngsuite():
     dirname = os.path.join(
         os.path.dirname(__file__),