Fix new field bug and support Python 3.13

[alexhad6]

Fixes a bug where defining new fields on a ParamDataclass would cause a KeyError. Also supports Python 3.13.

Also bumps the version to 0.15.3. To publish to PyPI, we'll need to create a GitHub release (see https://github.com/PainterQubits/paramdb/releases) after this is merged.

Also, the docs/readthedocs.org:paramdb CI check is supposed to build the docs website to make sure nothing broke it. I'm not sure the CI check is working anymore, but it should be fine to merge this anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.54%. Comparing base (6172548) to head (3f42e93).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   94.40%   94.54%   +0.14%     
==========================================
  Files           7        7              
  Lines         518      532      +14     
==========================================
+ Hits          489      503      +14     
  Misses         29       29              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.