Add getattr overload variants to help mypy type inference
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.
Manually cherry-picked from python/typeshed#6355.
