autodoc + classmethod coroutines do not play well together

Here's my code:
```python
class Test:
@classmethod
def a(cls):
"""yay"""
pass

@classmethod
async def b(cls):
"""yay"""
pass

async def c(self):
"""yay"""
pass

def d(self):
"""yay"""
pass
```

My doc:
```rst
.. autoclass:: module.Test
:members: a, b, c, d
```

Here's the generated rst:
```rst
.. py:class:: Test
:module: procrastinate.aiopg_connector

.. py:classmethod:: Test.a()
:module: procrastinate.aiopg_connector

yay


.. py:comethod:: Test.b()
:module: procrastinate.aiopg_connector

yay


.. py:comethod:: Test.c()
:module: procrastinate.aiopg_connector

yay


.. py:method:: Test.d()
:module: procrastinate.aiopg_connector

yay

```

Here's the output:

As you can see, b fails to be shown as an async class method.