In other words casting to bytes/unicode preserves pygolang string to
remain pygolang string.

Without the changes to bstr/ustr added test fails as e.g.

    >       assert bytes  (bs) is bs
    E       AssertionError: assert b'\xd0\xbc\xd0\xb8\xd1\x80' is b'\xd0\xbc\xd0\xb8\xd1\x80'
    E        +  where b'\xd0\xbc\xd0\xb8\xd1\x80' = bytes(b'\xd0\xbc\xd0\xb8\xd1\x80')

in other words bytes(bstr) was creating a copy and changing type to bytes.