betterproto v2.0.0b2 missing `List` in the generating code

We generated the following code:

```py
# ...
from typing import AsyncIterable, AsyncIterator, Iterable, Optional, Union
# ...
    async def contact_phone(
        self, *, contact_id: str = "", phone_list: Optional[List[str]] = None
    ) -> "puppet.ContactPhoneResponse":
# ...
```

As we can see: there's a `List` in the code, but there's no `List` imported from `typing`.