Update broken pandas links

[jameshadfield]

The previous URL is archived at
https://web.archive.org/web/20260306082838/https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#indexing-query but Panda's URL scheme seems to have changed with the release of v3.0. The "latest" version of the page is
https://pandas.pydata.org/docs/user_guide/indexing.html#indexing-query. Since we explicitly don't install pandas 3.0 the most recent v2 URL is preferable
https://pandas.pydata.org/pandas-docs/version/2.3/user_guide/indexing.html#indexing-query.

The broken link was identified in CI
https://github.com/nextstrain/augur/actions/runs/23122121288/job/67157947796 and this commit replaces all occurrences of it in this repo.

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.48%. Comparing base (3a74f0f) to head (451cf83).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1973   +/-   ##
=======================================
  Coverage   74.48%   74.48%           
=======================================
  Files          82       82           
  Lines        9124     9124           
  Branches     1859     1859           
=======================================
  Hits         6796     6796           
  Misses       2021     2021           
  Partials      307      307           

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