`backoff.on_predicate` does not work for `staticmethod`s

[spokeydokeys]

I'm decorating a staticmethod with backoff.on_predicate and when there is an error in the decorated methods I'm getting the error, TypeError: 'staticmethod' object is not callable. It seems that backoff is storing the static method unbound to the class and attempts to run the unbound descriptor rather than the embedded function.

I was able to get it to work locally by adding

target_fun = target.__func__ if isinstance(target, staticmethod) else target

in _sync.py (and doing something similar with the handlers).