Update actions to use actions/checkout@v3 and actions/setup-python@v4.

[tompollard]

The "run tests" GitHub workflow is raising several warnings about deprecated actions:

e.g.

[build (ubuntu-latest, 3.9)](https://github.com/MIT-LCP/wfdb-python/actions/runs/9783079800/job/27011340622) The following actions uses Node.js version which is deprecated and will be forced to run on node20: actions/checkout@v2, actions/setup-python@v2. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/[build (ubuntu-latest, 3.9)](https://github.com/MIT-LCP/wfdb-python/actions/runs/9783079800/job/27011340622) The following actions uses Node.js version which is deprecated and will be forced to run on node20: actions/checkout@v2, actions/setup-python@v2. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

This pull request updates the actions to the latest versions:

• actions/checkout@v2 -> actions/checkout@v3
• actions/setup-python@v2 -> actions/setup-python@v4

Note there is one outdated action that will need addressing at some point. As of today, the issue does not appear to have been addressed: actions/checkout#334

      # Note: "actions/checkout@v2" requires libstdc++6:amd64 to be
      # installed in the container.  To keep things simple, use
      # "actions/checkout@v1" instead.
      # https://github.com/actions/checkout/issues/334

[tompollard]

thanks!