fix: type annotation breaking on py3.9 (#28396)

superset/utils/pandas_postprocessing/contribution.py

@@ -14,6 +14,8 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+from __future__ import annotations
+
 from decimal import Decimal
 from typing import Any