Try to fix MyPy GitHub Actions (#1340)

.github/workflows/mypy.yml

@@ -20,4 +20,4 @@ jobs:
           # Set fetch-depth: 0 to fetch all history for all branches and tags.
           fetch-depth: 0 # Needed for setuptools_scm to work correctly
       - run: pip install -U --user pip setuptools setuptools-scm nox
-      - run: python -m nox --non-interactive --session validate-${{ python-version }} -k mypy  # Run nox for mypy
+      - run: python -m nox --non-interactive --session validate-3.13 -k mypy  # Run nox for mypy