Fix mypy failure

Author: yuanx749

pandas/core/dtypes/cast.py

@@ -1225,7 +1225,7 @@ def maybe_cast_to_datetime(
     _ensure_nanosecond_dtype(dtype)
 
     if lib.is_np_dtype(dtype, "m"):
-        if getattr(value, "ndim", 1) == 2 and value.shape[1] == 1:
+        if isinstance(value, np.ndarray) and value.ndim == 2 and value.shape[1] == 1:
             res = TimedeltaArray._from_sequence(value.ravel(), dtype=dtype)
             return res.reshape(value.shape)
         res = TimedeltaArray._from_sequence(value, dtype=dtype)