CI: Let pytest treat warnings as errors

Uses the  -Werror option of pytest to treat warnings as errors.

See https://docs.python.org/3/using/cmdline.html#cmdoption-W

.github/workflows/build_lint.yml

@@ -35,7 +35,7 @@ jobs:
   #   - name: Install dependencies
   #     run: pip install mesa pytest
   #   - name: Test with pytest
-  #     run: pytest test_examples.py
+  #     run: pytest -rA -Werror test_examples.py
 
   build-pre:
     runs-on: ubuntu-latest
@@ -50,7 +50,7 @@ jobs:
         pip install mesa --pre
         pip install pytest
     - name: Test with pytest
-      run: pytest -rA test_examples.py
+      run: pytest -rA -Werror test_examples.py
 
   build-main:
     runs-on: ubuntu-latest
@@ -65,4 +65,4 @@ jobs:
         pip install pytest
         pip install -U git+https://github.com/projectmesa/mesa@main#egg=mesa
     - name: Test with pytest
-      run: pytest -rA test_examples.py
+      run: pytest -rA -Werror test_examples.py