Bump to 0.40.0

[fcollonval]

⚠️ This is changing the previous rules

JLab 2 <-> git extension 0.2#.#
JLab 3 <-> git extension 0.3#.#

But it was not a great idea and I prefer to follow semver.

So 0.40.0 will be compatible with JupyterLab 3 (and only 3 as some changes will be required for JLab 4).

[github-actions]

👈 Launch a binder notebook on branch jupyterlab/jupyterlab-git/fcollonval-patch-1