BUG: 'ExponentialMovingWindow' object has no attribute 'apply' when using 'agg'
The following code fails:

```python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.normal(size=(100, 10)))
df.ewm(alpha=0.1).agg(lambda x: x)

```

Below the traceback

```python
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-69-c018ca7d52b8> in <module>
----> 1 df.ewm(alpha=0.1).agg(lambda x: x)

c:\dev\projects\weedget\venv\lib\site-packages\pandas\core\window\ewm.py in aggregate(self, func, *args, **kwargs)
    316     )
    317     def aggregate(self, func, *args, **kwargs):
--> 318         return super().aggregate(func, *args, **kwargs)
    319 
    320     agg = aggregate

c:\dev\projects\weedget\venv\lib\site-packages\pandas\core\window\rolling.py in aggregate(self, func, *args, **kwargs)
    472         result, how = aggregate(self, func, *args, **kwargs)
    473         if result is None:
--> 474             return self.apply(func, raw=False, args=args, kwargs=kwargs)
    475         return result
    476 

c:\dev\projects\weedget\venv\lib\site-packages\pandas\core\window\rolling.py in __getattr__(self, attr)
    206             return self[attr]
    207 
--> 208         raise AttributeError(
    209             f"'{type(self).__name__}' object has no attribute '{attr}'"
    210         )

AttributeError: 'ExponentialMovingWindow' object has no attribute 'apply'
```
