contextlib.pyi ExitStack.callback take Callable (#1908)
contextlib.pyi ExitStack.callback take Callable (#1908)

* contextlib.pyi ExitStack.callback take Callable

The documentation for ExitStack does not specify the return type of the callable's passed to callback.  
Saying that they always return None breaks a very common use case for ExitStack, which is to "schedule" changes to a collection 
while iterating it.  

```
with ExitStack() as stack:
    for key, value in adict.items():
        if ...:
            stack.callback(adict.pop, key)
```

I also added AsyncExitStack, using Awaitable.

* Moving to Explicit Any in Generics

euresti/typeshed,li-dan/typeshed