As per the comment at https://bugs.python.org/issue32445#msg309356, there's a bug in my suggested changes to `ExitStack.pop_all()`: the right method to call is ExitStack.push(), *not* ExitStack.callback() (the latter adds a wrapper function to make the signatures match, but our stored callbacks all already have the right signature).

I'm not too fussy about the details of the docstring, but we need to be careful about the guarantees we make to ExitStack subclasses: if the docstring implies that "target.push()" will always be called, then we need to limit the stack stealing behaviour to actual ExitStack instances (which may be a good idea anyway).