BUG: SparseDtype requires numpy dtype
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
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- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Seems like a user was relying on passing a sparse categorical type, does it mean it needs deprecating? https://github.com/pandas-dev/pandas/issues/49987 > Seems like a user was relying on passing a sparse categorical type, does it mean it needs deprecating? https://github.com/pandas-dev/pandas/issues/49987

They were, but then it would silently give them an unwanted cast to Sparse[object]

```
df = pd.DataFrame([["a", 0],["b", 1], ["b", 2]], columns=["A","B"])
>>> df["A"].astype(SparseDtype("category"))
Sparse[object, nan]
``` Thanks @jbrockmendel 