Inconsistency with the Python version for the macos CI checks

[jtpio]

Description

At first this looks more like an issue with GitHub Actions but opening here for awareness.

There seems to be some inconsistencies with the Python version installed with actions/setup-python and the one actually used later in the workflows:

with macos

with linux

Reproduce

With the latest commit on master: https://github.com/jupyterlab/jupyterlab/runs/7968878969?check_suite_focus=true

Expected behavior

The Python version should be the one set with the actions/setup-python action.

Context

This has also been noticed in other places:

• Troubleshoot docs build voila-dashboards/voila#1190 (comment)
• Drop Python 3.6, update to ipywidgets 8 jtpio/ipylab#100 (comment)

[jtpio]

Opened actions/setup-python#487.