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>

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

@@ -28,6 +28,7 @@ class PyOnnx(PythonPackage):
     depends_on('py-six', type=('build', 'run'))
     depends_on('py-typing@3.6.4:', type=('build', 'run'))
     depends_on('py-typing-extensions@3.6.4:', type=('build', 'run'))
+    depends_on('cmake@3.1:', type='build')
 
     # 'python_out' does not recognize dllexport_decl.
     patch('remove_dllexport_decl.patch', when='@:1.6.0')