Fix pytest-weekly Action script

[fpavogt]

Description:

This PR fixes the Github Action script responsible for running pytest automatically at regular interval (the default Python version was incorrectly=not set). It also updates the pylint Action following a deprecation in pylint 3.x

Error(s) fixed:
N.A.

Checklists:

• New code includes dedicated tests.
• New code has been linted.
• New code follows the project's style.
• New code is compatible with the 3-Clause BSD license.
• CHANGELOG has been updated.
• AUTHORS has been updated.
• Copyright years in module docstrings have been updated.

[fpavogt]

Hey @loforest, @regDaniel, @pjuda,

FYI, I'll merge this today so we can check tonight if the script runs ok. This PR is only touching a Github Action.

[pjuda]

Approved.

[regDaniel]

Tiptop, thanks!