message: Update README for new test_requires
diff: @@ -88,7 +88,7 @@ Change into another directory to fix this error.
  ONNX uses [pytest](https://docs.pytest.org) as test driver. In order to run tests, first you need to install pytest:
  ```
  -pip install pytest-cov
  +pip install pytest-cov nbval
  ```
  After installing pytest, do
  