Proper way to specify type annotation of a MapAttribute subclass?

Supposed I define a `MapAttribute` subclass like follows:
```
class MyMapAttribute(MapAttribute):
    pass
```

When I run pre-commit's (rev 2.1.0) mypy hook (rev 0.701), I run into the following error:

```
error: Missing type parameters for generic type
```

I am using `pynamodb==4.0.0`.