Adjust for pandas v3.0.0

[khaeru]

Pandas 3.0.0 was released 2026-01-21, causing failures in the test suite, e.g. here.

A fix for one failed test is upstream: khaeru/sdmx#268.

The following failure is pandas-dev/pandas#63876:

FAILED transport_data/tests/test_jrc.py::test_convert - AttributeError: 'StringArray' object has no attribute 'item'

This PR adjusts the code to work around the issue.
If it is resolved upstream in a future pandas version, the workaround may be removed.

PR checklist

• Checks all ✅
• Update documentation N/A, bug fix only.
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (9d43918) to head (2264135).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   93.89%   96.15%   +2.25%     
==========================================
  Files          66       66              
  Lines        3475     3145     -330     
==========================================
- Hits         3263     3024     -239     
+ Misses        212      121      -91     

Files with missing lines	Coverage Δ
transport_data/jrc/__init__.py	93.40% <100.00%> (+0.14%)	⬆️

... and 2 files with indirect coverage changes

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