groupby fails for pyhon 3 unsortable objects with sort=True
The default groupby behaviour will try to sort every object. But python 3 does not define an ordering on everything and so the method will fail with the default parameters. In python 2 the order of objects is random (the id).

``` python

import pandas as pd

class A: pass

df = pd.DataFrame([[A(), i, i**2] for i in range(10)])

for x in df.groupby(0,1):
    print(x)
```
