Update actions for Node.js 16 deprecation

[AdamOrmondroyd]

Description

github actions have deprecated Node.js 16 in favour of Node.js 20, which requires us to update the versions of the checkout and setup-python actions to their latest versions.

Fixes # (issue)

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 anesthetic tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works
• I have appropriately incremented the semantic version number in both README.rst and anesthetic/_version.py

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b93dec5) 100.00% compared to head (3947c38) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #361   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        34           
  Lines         2978      2978           
=========================================
  Hits          2978      2978           

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[williamjameshandley]

Seems very reasonable. Many thanks @AdamOrmondroyd