run mypy in github ci correctly

Author: Phil Varner

.github/workflows/pr.yml

@@ -24,6 +24,8 @@ jobs:
         run: |
           python -m pip install pre-commit
           pre-commit run --all-files
-          mypy src
+      - name: Type-check code
+        if: ${{ matrix.python-version == 3.12 }}
+        run: poetry run mypy src
       - name: Run tests
         run: poetry run nox