Update GitHub actions

[lfarv]

• The action setup-python is updated v5 to avoid a warning: Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/setup-python@v4.
• The flake8 options are modified for compatibility with the black code formatter. In particular, the recommended line length is set to 88,
• pytest is restricted to release <8 because the new version 8.0.0 make the test sequence crash.

Note that the Matlab actions (setup-matlab and run-command) are not yet upgraded to use Node.js 20, so they still trigger the warning.

[lfarv]

The problem with pytest has been solved in #735, so it's no more mentioned here.

@swhite2401 , @MJGaughran : this PR is not critical, but I would like to move on. Do you have comments?