DOC: Fix attribution for testing function

The testing functions taken from scikit-learn were in a file with a copyright assigned to Pietro, but Andreas was resposible for the specific lines that were copied.

skimage/_shared/testing.py

@@ -1,7 +1,5 @@
 """Testing utilities."""
 
-# Copyright (c) 2011 Pietro Berkes
-# License: Simplified BSD
 
 def _assert_less(a, b, msg=None):
     message = "%r is not lower than %r" % (a, b)