DO NOT MERGE: Test enforce nightly

[OFR-IIASA]

This PR tests to see if a fix of models.py results in all nightly tests passing.

DO NOT MERGE DESPITE BEING APPROVED

[codecov]

Codecov Report

Merging #612 (3b224d4) into main (b572cce) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #612   +/-   ##
=====================================
  Coverage   92.8%   92.9%           
=====================================
  Files         42      43    +1     
  Lines       3287    3386   +99     
=====================================
+ Hits        3052    3147   +95     
- Misses       235     239    +4     

Impacted Files	Coverage Δ
message_ix/message_ix/models.py	96.8% <0.0%> (-3.2%)	⬇️
message_ix/message_ix/core.py	95.1% <0.0%> (-0.1%)	⬇️
message_ix/message_ix/tests/test_core.py	100.0% <0.0%> (ø)
..._ix/tests/test_feature_vintage_and_active_years.py	100.0% <0.0%> (ø)

[gidden]

Approving review to trigger nightlies

[khaeru]

This fix was implemented and merged in #613.