API Change for arithmetic methods for non-pandas objects
Possible API breakage for dask in https://github.com/pandas-dev/pandas/pull/16821

```python
import dask.dataframe as dd
import pandas as pd

from dask.dataframe.utils import assert_eq


s = dd.core.Scalar({('s', 0): 10}, 's', 'i8')
pdf = pd.DataFrame({'a': [1, 2, 3, 4, 5, 6, 7],
                    'b': [7, 6, 5, 4, 3, 2, 1]})
result = (pdf + s).dtypes  # This now casts to object, used to retain int64
expected = pdf.dtypes
assert_eq(result, expected)
```

In master, `result` is now `(object, object)`. Before it was `(int64, int64)`.

I'm looking into #16821 to see if this was unintentional, and can be avoided.