collect_snippets prints warnings to console

[lutzroeder]

This code...

from onnx.backend.test.case import collect_snippets
snippets = collect_snippets()

...prints these warnings when run with Python 3.7.4:

./onnx/lib/python3.7/site-packages/onnx/backend/test/case/node/scatter.py:35: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.
  indices[make_slice(indices, axis, i)].reshape(1, -1)[0]] for i in range(indices.shape[axis])]
./onnx/lib/python3.7/site-packages/onnx/backend/test/case/node/scatter.py:45: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.
  scattered[idx] = updates[updates_idx]
./onnx/lib/python3.7/site-packages/onnx/backend/test/case/node/scatterelements.py:34: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.
  indices[make_slice(indices, axis, i)].reshape(1, -1)[0]] for i in range(indices.shape[axis])]
./onnx/lib/python3.7/site-packages/onnx/backend/test/case/node/scatterelements.py:44: FutureWarning: Using a non-tuple sequence for multidimensional indexing is deprecated; use `arr[tuple(seq)]` instead of `arr[seq]`. In the future this will be interpreted as an array index, `arr[np.array(seq)]`, which will result either in an error or a different result.
  scattered[idx] = updates[updates_idx]

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