dedup does not work with logically equivalent expressions

```
>>> from license_expression import Licensing
>>> lic_exp = "(apache-2.0 OR mit) AND (mit OR apache-2.0)"
>>> str(Licensing().dedup(lic_exp))
'(apache-2.0 OR mit) AND (mit OR apache-2.0)'
```

The `apache-2.0 OR mit` and `mit OR apache-2.0` is technically equivalent  and should be deduplicated.