Migrate to pyproject.toml instead of setup.cfg

[mwcraig]

Please have a look at the following list and replace the "[ ]" with a "[x]" if
the answer to this question is yes.

Please note that the last point is not a requirement. It is meant as a check if
the pull request potentially breaks backwards-compatibility.

---

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (a7a6d8b) to head (b266531).
Report is 62 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #832      +/-   ##
==========================================
+ Coverage   97.61%   97.75%   +0.14%     
==========================================
  Files           9        9              
  Lines        1383     1383              
==========================================
+ Hits         1350     1352       +2     
+ Misses         33       31       -2     

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