Update ipywidgets packaging instructions

Author: jasongrout

docs/source/dev_release.md

@@ -87,7 +87,8 @@ curl -s https://pypi.org/pypi/widgetsnbextension/json | jq  -r '[.releases[][] |
 Change `ipywidgets/_version.py` to reflect the new version number, and if necessary, a new `__html_manager_version__`. Change the `install_requires` parameter in `setup.py` reference the new widgetsnbextension version.
 
 ```
-python setup.py sdist bdist_wheel
+pip install build
+python -m build
 twine upload dist/*
 ```