Adjust for pandas 1.5.0

[khaeru]

Closes #457.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• ~Add, expand, or update documentation.~
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #458 (3fb9316) into main (a5a64d2) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #458   +/-   ##
=====================================
  Coverage   98.6%   98.6%           
=====================================
  Files         42      42           
  Lines       4460    4467    +7     
=====================================
+ Hits        4398    4405    +7     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/tests/test_utils.py	100.0% <ø> (ø)
ixmp/__init__.py	100.0% <100.0%> (ø)
ixmp/backend/io.py	98.5% <100.0%> (ø)
ixmp/backend/jdbc.py	95.2% <100.0%> (+<0.1%)	:arrow_up:
ixmp/tests/backend/test_base.py	98.9% <100.0%> (ø)
ixmp/tests/backend/test_jdbc.py	100.0% <100.0%> (ø)
ixmp/utils.py	99.5% <100.0%> (-0.1%)	:arrow_down: