These silence errors about missing type annotations for calls
like these:
```
x = getattr(o, 'a', [])
y = getattr(o, 'b', {})
```
This is basically a generalization of #5518 and other overloads we already
have.
This works around python/mypy#11572. I encountered the issue in several
places when testing recent typeshed against an internal repo.
As brought up in https://github.com/python/typeshed/pull/5516
Alternatives include:
- Use another type var that has a value restriction
- Doing something fancy with Protocols that have a __bool__ that returns
a Literal (which may not work)
- Doing nothing
