Support PEP660 - editable installation

[fcollonval]

As PEP660 has been accepted for editable installation, it will be nice to add it to jupyter-packaging (and to figure out how we could do editable installation for JLab frontend extension).

Ref: https://www.python.org/dev/peps/pep-0660/

[blink1073]

Once pypa/setuptools#2872 is merged and released we can extend the new build functions in the same way we currently do with the other functions in setuptools.build_meta.

[blink1073]

pypa/setuptools#2872 was closed, and pypa/setuptools#3082 was opened in its stead, but it may be a while before support lands in setuptools.