Update test_algos.py

support python3 syntax

Author: mattip

pandas/tests/test_algos.py

@@ -1082,7 +1082,7 @@ def _test_vector_resize(htable, uniques, dtype, nvals, safely_resizes):
             else:
                 with pytest.raises(ValueError) as excinfo:
                     htable.get_labels(vals, uniques, 0, -1)
-                assert excinfo.value.message.startswith('external reference')
+                assert str(excinfo.value).startswith('external reference')
             tmp2 = uniques.to_array()
             assert tmp.shape == oldshape