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

In the following,
```python
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`.