fix pandas applymap deprecation warning

[atravitz]

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit by making a comment with pre-commit.ci autofix before requesting review.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.01%. Comparing base (f0f968f) to head (e84f42c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1701      +/-   ##
==========================================
- Coverage   95.41%   93.01%   -2.41%     
==========================================
  Files         185      185              
  Lines       16045    16045              
==========================================
- Hits        15310    14924     -386     
- Misses        735     1121     +386     

Flag	Coverage Δ
fast-tests	93.01% <100.00%> (?)
slow-tests	?

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