BUG: my_DataFrame.groupby().min()/-max() gives Assertion error message
With pandas 1.0.0. (and newer) min/max methods for groups to not work as before:
```python
my_DataFrame.groupby().min()
my_DataFrame.groupby().max()
```
are not working anymore - it will give you an Assertion error message with no special location in the code.

You currently new to use aggregation to get this information:
```python
my_DataFrame.groupby().agg('min')
my_DataFrame.groupby().agg('max')
```

I could verify that
```python
my_DataFrame.groupby().mean()
```
still works in newest pandas, but somehow -min()/-max() no more.

Would it be possible to implement these methods back again to pandas?
I can confirm that my_DataFrame.groupby().max()/-min() worked properly in pandas 0.3.0.

Kind regards,
DK