`backoff.expo` accepts floats. Most usage in our codebase has a floating point `factor`. If we want to preserve exact inference in cases where the return type is `Generator[int, Any, None` we could use an overload (TypeVar could work, but would be a little worse for cases involving defaults).

This generators will work fine with floats for arguments. In the type
hinting world, float can be used to mean int or float.

If needed, I want to make sure we put this in the right place. There
may ultimately be some other types which it would be helpful to expose
as well, and in that case I think it might be better to have a typing
submodule where only these types are exposed.