Deprecate Series.ftype / DataFrame.ftypes
With the `ftype(s)` property, you could check if data was sparse:

```
In [26]: df = pd.DataFrame({'a': pd.SparseArray([1, np.nan, 1])})

In [27]: df
Out[27]: 
     a
0  1.0
1  NaN
2  1.0

In [28]: df.dtypes
Out[28]: 
a    Sparse[float64, nan]
dtype: object

In [29]: df.ftypes
Out[29]: 
a    float64:sparse
dtype: object
```

But now that the dtype itself is a EA SparseDtype, I don't think those `ftype` and `ftypes` attributes serve any good purpose. So let's deprecate them.