TST: Add test for categorical with str and tuples

The bug is not present any more.

closes #21416

Author: has2k1

pandas/tests/arrays/categorical/test_constructors.py

@@ -601,3 +601,9 @@ def test_constructor_imaginary(self):
         c1 = Categorical(values)
         tm.assert_index_equal(c1.categories, Index(values))
         tm.assert_numpy_array_equal(np.array(c1), np.array(values))
+
+    def test_constructor_string_and_tuples(self):
+        #GH 21416
+        c = pd.Categorical(['c', ('a', 'b'), ('b', 'a'), 'c'])
+        expected_index = pd.Index([('a', 'b'), ('b', 'a'), 'c'])
+        assert c.categories.equals(expected_index)