Address failing pytest

[LauWien]

This PR addresses the failing _pytest.yam_l. It tries out if the test_add_year.py is still failing using the new pandas release v1.4.1.

Update: the tests in test_add_year.py succeed. Tests in test_tutorials.py are failing in

• every OS with Python version 3.6,
• Python version 3.8 and 3.9 for ubuntu-latest
  For macOS latest and Windows latest all tests are successful on Python version 3.8 and 3.9. Please see here.

How to review

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

PR checklist

• Continuous integration checks all ✅

[codecov]

Codecov Report

Merging #565 (6c3ef88) into main (1df3619) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #565     +/-   ##
=======================================
- Coverage   93.8%   93.7%   -0.1%     
=======================================
  Files         40      40             
  Lines       3057    3057             
=======================================
- Hits        2870    2867      -3     
- Misses       187     190      +3     

Impacted Files	Coverage Δ
message_ix/tests/test_tutorials.py	88.4% <0.0%> (-11.6%)	⬇️

[LauWien]

Supersede by #557. See also conversations in #566.