use newer version of actions/checkout and actions/setup-python, to avoid relying on deprecated node.js 16

[boegel]

fix for warning popping up in GitHub Actions:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, actions/setup-python@v4. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

[boegel]

The tests that use a CentOS 7.9 container are failing, because node.js 20 doesn't work there:

 /__e/node20/bin/node: /lib64/libm.so.6: version `GLIBC_2.27' not found (required by /__e/node20/bin/node)

So this may have to wait until we no longer need to test in CentOS 7.9 (i.e. when we've collapsed the 5.0.x branch into develop in preparation for releasing EasyBuild v5.0)