Use branch for hgrecco/pint#1508 in pytest CI workflow

.github/workflows/pytest.yaml

@@ -60,7 +60,7 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
-    - name: Get pip cache dir
+    - name: Locate pip cache directory
       id: pip-cache
       run: echo "::set-output name=dir::$(pip cache dir)"
 
@@ -78,10 +78,14 @@ jobs:
       run: python -m pip install --upgrade pip wheel setuptools-scm
 
     - name: Install Python package and dependencies
-      # pyam-iamc (IAMconsortium/pyam#589) forces pint 0.17; override
       run: |
         pip install --editable .[docs,tests]
-        pip install --upgrade pint
+
+    - name: Specify version of pint
+      if: matrix.python-version != '3.7'
+      run: |
+        # Branch for with https://github.com/hgrecco/pint/pull/1508
+        pip install --upgrade git+https://github.com/khaeru/pint.git@issue/1498#egg=pint
 
     - name: Run test suite using pytest
       run: pytest genno --trace-config --verbose --cov-report=xml --cov-report=term --color=yes