tests: Fix up pickle tests

tests/test_pickle.py

@@ -195,14 +195,14 @@ def test_internal_symbols(self, pickle):
         assert new_s.name == s.name
         assert new_s.dtype is np.int32
         assert new_s.is_const is True
-        assert new_s.nonnegative is None
+        assert new_s.is_nonnegative is None
 
         s = Scalar(name='s', nonnegative=True)
         pkl_s = pickle.dumps(s)
         new_s = pickle.loads(pkl_s)
         assert new_s.name == s.name
         assert new_s.assumptions0['nonnegative'] is True
-        assert new_s.nonnegative is True
+        assert new_s.is_nonnegative is True
 
     def test_bound_symbol(self, pickle):
         grid = Grid(shape=(3, 3, 3))