Series.sort_values issue when values are data types
```python
import pandas as pd
import numpy as np

n = 100

df = pd.DataFrame({"a": np.random.randn(n),
                   "b": ["2018-01-01"] * n})

df[["c", "d"]] = df

# works as expected
df.dtypes.sort_values()

df["b"] = pd.to_datetime(df["b"])
df["d"] = pd.to_datetime(df["d"])

# the datetimes are not grouped together
df.dtypes.sort_values()
```

It looks like the `Series` method `sort_values` might have an issue when the `Series` values are data types that contain datetimes.  The expected output would be to have the two `datetime64[ns]` values grouped together.