Enable python-mode expansions for expand-region.

Without this, the following Python (with point shown by |):

    class PushTest(PreludeTestCase):
        def test_push_list(self):
            expected = List([Integer(1)])
            self.assertEvalsTo(u"(set-symbol! (quote x) (quote ())) (push! x 1) x", expected)

        def test_push_bytestring(self):
            self.assertEqual(
                evaluate_all_with_prelude(parse(lex(
                    u'(set-symbol! |(quote x) #bytes("bc")) (push! x 97) x'))),
                Bytestring([ord(c) for c in b"abc"]))

does not expand the unicode single-quoted string correctly.