Different Yarn versions in package.json and dev_install files

[jorgepiloto]

Problem

The dev_install asks for Yarn>=1.2.1:

ipywidgets/docs/source/dev_install.md

Line 10 in bb2edf7

- [yarn](https://yarnpkg.com/) package manager ** version 1.2.1 or later **

However, the package.json needs Yarn>=3.0.0:

ipywidgets/package.json

Line 63 in bb2edf7

"yarn": ">=3"

Suggested Improvement

The dev_install.md file should be updated to reflect the same version as the one in package.json.