TST: adjust for fixup in diffpy.structure

Test passes with diffpy.structure 3.0.1.

Author: pavoljuhas

src/diffpy/srfit/tests/testpdf.py

@@ -270,11 +270,6 @@ def test_pickling(self):
         self.assertTrue(numpy.array_equal(res1, pc2.residual()))
         return
 
-    # TODO: remove after fixup release of diffpy.structure
-    import sys
-    if sys.version_info >= (3, 7):
-        test_pickling = unittest.expectedFailure(test_pickling)
-
 # End of class TestPDFContribution
 
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