[MNT] Use walrus operator in study.py to resolve TODO

[shellyco-code]

Description

This PR resolves an outstanding TODO comment in openml/study/study.py by refactoring legacy variable assignments to use the Python 3.8+ walrus operator (:=).

This change is now safe to make as the project has updated its requires-python specification to >=3.10 in pyproject.toml, officially dropping Python 3.7 support.

Resolves #1746

Changes

• Replaced two instances of getattr followed by an is not None check with the walrus operator in openml/study/study.py.
• Removed the old # TODO(eddiebergman): Begging for a walrus if we can drop 3.7 comments.

Checklist

• I have read the CONTRIBUTING guide.
• I have run the pre-commit checks locally (pre-commit run --all-files).
• All tests related to my changes pass locally (pytest tests).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.32%. Comparing base (fabbc28) to head (b5c8350).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1747      +/-   ##
==========================================
- Coverage   81.50%   81.32%   -0.18%     
==========================================
  Files          63       63              
  Lines        5240     5238       -2     
==========================================
- Hits         4271     4260      -11     
- Misses        969      978       +9     

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