Thanks for the patch, David!

+    def test_fromlist_error_messages(self):
+        # Test for issue #21720: fromlist unicode error messages
+        try:
+            __import__('encodings', fromlist=[u'aliases'])
+        except TypeError as exc:
+            self.assertIn("must be str, not unicode", str(exc))

You could use assertRaises here:

    with self.assertRaises(TypeError) as cm:
        # ...

    self.assertIn('foo', str(cm.exception))


+        if (PyUnicode_Check(item)) {
+            PyErr_SetString(PyExc_TypeError,
+                            "Item in ``from list'' must be str, not unicode");
+            Py_DECREF(item);
+            return 0;
+        }

I think it would be better to improve the error message in Python/import.c:

    http://hg.python.org/cpython/file/2.7/Python/import.c#l2571

So you can safely remove this check.