fix(requirements): lock onnx version

[AmitMY]

Another fix for the same issue was already merged: https://github.com/gmalivenko/pytorch2keras/pull/133

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See: https://github.com/onnx/onnx/issues/582#issuecomment-824263936

As of 2021 March 15th, the optimizer is no longer part of the onnx repo (source).

If you're installing code that depends on it and get errors, an easy workaround is to install a slightly older version of onnx:

pip install onnx==1.8.1

Please note that the onnxoptimizer is still available and ongoing from another repo: https://github.com/onnx/optimizer. As mentioned above, it was moved out from ONNX package since ONNX 1.9.0. Thanks