1.6.2: frozenlist does not work with deepcopy

I introduced the problem in 1.6.2 for using the dependency_injector library.

dependency_injector creates a deep copy for you (maybe someday, when and why, it will happen).

How to check:
```python
from copy import deepcopy
import frozenlist from FrozenList

deepcopy(FrozenList([1, 2, 3]))
```

Error: `TypeError: self._frozen cannot be converted to a Python object for pickling`