Work around pydata/pandas-dev#39168

[khaeru]

Closes #390.

How to review

Note that the CI checks all pass.

PR checklist

• Add or expand tests. N/A
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #391 (7ca00cd) into master (62f07ce) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #391     +/-   ##
========================================
- Coverage    96.8%   96.8%   -0.1%     
========================================
  Files          46      46             
  Lines        5334    5334             
========================================
- Hits         5168    5167      -1     
- Misses        166     167      +1     

Impacted Files	Coverage Δ
ixmp/tests/core/test_timeseries.py	100.0% <100.0%> (ø)
ixmp/utils.py	95.8% <0.0%> (-0.5%)	⬇️