DM-35347: Switch to pyproject.toml

[mwittgen]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #196 (494a594) into main (1ad71ff) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 494a594 differs from pull request most recent head db850e5. Consider uploading reports for the commit db850e5 to get more accurate results

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   82.45%   82.42%   -0.04%     
==========================================
  Files          47       47              
  Lines        3751     3750       -1     
  Branches      675      675              
==========================================
- Hits         3093     3091       -2     
- Misses        480      481       +1     
  Partials      178      178              

Impacted Files	Coverage Δ
python/lsst/ctrl/mpexec/taskFactory.py	84.37% <0.00%> (-2.73%)	⬇️
python/lsst/ctrl/mpexec/cmdLineFwk.py	61.93% <0.00%> (ø)
python/lsst/ctrl/mpexec/cli/opt/options.py	100.00% <0.00%> (ø)
python/lsst/ctrl/mpexec/cli/script/qgraph.py	40.00% <0.00%> (ø)
python/lsst/ctrl/mpexec/cli/opt/optionGroups.py	100.00% <0.00%> (ø)

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[timj]

It looks like there are mypy failures from the recent changes to pex_config upstream. @natelust might be able to help you fix those if it's not clear.