test pinning pillow

.github/workflows/python-package.yml

@@ -23,6 +23,7 @@ jobs:
         astropy-version: ['<5.0', '<5.1']
         numpy-version: ['<1.20', '<1.21', '<1.22']
         scipy-version: ['<1.6', '<1.7']
+        # matplotlib >= 3.3 brings in a dependency on Pillow, which requires numpy >= 1.21 (unless it's pinned. Maybe Pillow <10.1?)
         matplotlib-version: ['<3.3'] # '<3.4', '<3.5']
 
     steps:
@@ -71,7 +72,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip wheel setuptools
           python -m pip install pytest pytest-astropy pyyaml coveralls
-          python -m pip install numpy\<1.21 scipy\<1.6 matplotlib\<3.3 astropy\<5.0
+          python -m pip install Pillow\<10.1 numpy\<1.21 scipy\<1.6 matplotlib\<3.4 astropy\<5.0
 
       - name: Run the test with coverage
         run: pytest --cov