Describe generic classes as base classes

The PEP should probably mention what happens if a generic class is used as a base class.

Examples to consider:

```
class StrList(List[str]): ...  # StrList is not generic

T = TypeVar('T')

class MyList(List[T]): ...   # Mypy requires Generic[T] as a base class, but it's ugly

x = Undefined(MyList[int])
```
