Override pyam-iamc pin to old Pint on Python 3.13

Author: khaeru

.github/workflows/pytest.yaml

@@ -107,6 +107,10 @@ jobs:
         # commented: use with "pandas-version" in the matrix, above
         # pip install --upgrade pandas${{ matrix.pandas-version }}
 
+        # TEMPORARY With Python 3.13 pyam-iamc resolves to 1.3.1, which in turn
+        # limits pint < 0.17. Override. cf. iiasa/ixmp#544
+        pip install --upgrade pint
+
     - name: Install R dependencies and tutorial requirements
       run: |
         install.packages(c("remotes", "Rcpp"))