ci: update versions of GitHub actions and pre-commit

[alexander-held]

Updating GitHub actions:

• setup-python v4: https://github.com/actions/setup-python/releases/tag/v4.0.0
• pre-commit v3: https://github.com/pre-commit/action/releases/tag/v3.0.0
• codecov v3: https://github.com/codecov/codecov-action/releases/tag/v3.1.0

Also including a pre-commit update.

* updated versions of GitHub actions: setup-python v4, pre-commit v3, codecov v3
* use strings for Python versions in CI
* updated pre-commit

[codecov]

Codecov Report

Merging #343 (0df4987) into master (fce7208) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #343   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1878      1878           
  Branches       299       299           
=========================================
  Hits          1878      1878           

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