@qcore.retry() doesn't work on classmethods

[JelleZijlstra]

In the following,

class X:
    @qcore.retry(KeyError)
    @asynq.async()
    @classmethod
    def f(cls): pass
    @qcore.retry(Exception)
    @classmethod
    def g(cls): pass

Neither X.f nor X.g can be called any more.

Fixing this probably requires reimplementing @retry using qcore.decorators.

[SurenNihalani]

Feel free to submit a PR!