Adjust 1 test for pyam-iamc ≥ 3

[khaeru]

pyam-iamc versions ≥ 3 do not sort data on the 'region' and 'variable' dimensions. This caused test/CI failures on Python 3.10 and later, but not Python 3.9, as pyam-iamc ≥ 3 was not released for this version of Python.

PR checklist

• Checks all ✅
• Update documentation N/A, no change in behaviour
• Update doc/whatsnew.rst N/A, test suite only

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (b1b7941) to head (e70499c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #155   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          52       52           
  Lines        4705     4708    +3     
=======================================
+ Hits         4698     4701    +3     
  Misses          7        7           

Files with missing lines	Coverage Δ
genno/tests/compat/test_pyam.py	100.00% <100.00%> (ø)