Update op_pool_common.py, add missing "| None" (onnx#6421)

for example

onnx/backend/test/case/node/averagepool.py
uses 
pads = None

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---------

Signed-off-by: Andreas Fehlner <[email protected]>

onnx/reference/ops/op_pool_common.py

@@ -51,7 +51,7 @@ def get_pad_with_auto_pad(auto_pad: str, pad_shape: Sequence[int]) -> Sequence[i
 
 
 def get_output_shape_explicit_padding(
-    pads: Sequence[int],
+    pads: Sequence[int] | None,
     input_spatial_shape: Sequence[int],
     kernel_spatial_shape: Sequence[int],
     strides_spatial: Sequence[int],