DataFrame.mean includes datetime64[ns] dtypes in 0.24.0rc1
I think this isn't deliberate.

```python
# 0.23.4
In [1]: import pandas as pd

In [2]: pd.DataFrame({"A": pd.date_range('2000', periods=12)}).mean()
Out[2]: Series([], dtype: float64)
```

```python
# 0.24.0rc1
In [2]: pd.DataFrame({"A": pd.date_range('2000', periods=12)}).mean()
Out[2]:
A   2124-07-04 18:21:24.687506176
dtype: datetime64[ns]
```

We continue to raise on `Series[datetime64[ns]].mean()`