Without explicitly overriding __reduce_ex__ pickling was failing for
protocols < 2:

    _________________________ test_strings_pickle __________________________

        def test_strings_pickle():
            bs = b("мир")
            us = u("май")

            #from pickletools import dis
            for proto in range(0, pickle.HIGHEST_PROTOCOL):
    >           p_bs = pickle.dumps(bs, proto)

    golang/golang_str_test.py:282:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = b'\xd0\xbc\xd0\xb8\xd1\x80', proto = 0

        def _reduce_ex(self, proto):
    >       assert proto < 2
    E       RecursionError: maximum recursion depth exceeded in comparison

    /usr/lib/python3.9/copyreg.py:56: RecursionError

See added comments for details.