Remove pint upgrade (#57)

Co-authored-by: Paul Natsuo Kishimoto <mail@paul.kishimoto.name>

.github/workflows/pytest.yaml

@@ -80,14 +80,11 @@ jobs:
     - name: Install Python package and dependencies
       run: |
         pip install --editable .[docs,tests]
-        # pyam-iamc (IAMconsortium/pyam#651) forces pint < 0.19; override
-        pip install --upgrade pint
         # TEMPORARY downgrade xarray pending khaeru/genno#67
         pip install xarray!=2022.6.0
         # TEMPORARY downgrade matplotlib pending has2k1/plotnine#619
         pip install "matplotlib<3.6.0"
 
-
     - name: Run test suite using pytest
       run: pytest genno --trace-config --verbose --cov-report=xml --cov-report=term --color=yes