BUG: Groupby.transform('cumcount') fails
Adding tests for #27467 surfaced some bugs:

```python
from pandas import *
df = DataFrame(dict(a=[0, 0, 0, 1, 1, 1], b=range(6)))
g = df.groupby(np.repeat([0,1], 3))
# ok
g.cumcount()
# fails
g.transform('cumcount')
AttributeError: ("'Series' object has no attribute 'cumcount'", 'occurred at index a')
```

`cumcount` is a cythonized function.