py-onnx: depends on cmake >= 3.1 (#15762)

* py-onnx: depends on cmake >= 3.1

* Update var/spack/repos/builtin/packages/py-onnx/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>