Fix mypy CI job (#498)

.github/workflows/check.yml

@@ -35,11 +35,15 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: ${{ matrix.python-version }}
+          # Need to use Python 3.8 for the mypy-3.8 job,
+          # or `ast_decompiler` isn't installed by `uv pip install -e '.[dev]'`,
+          # and `ast_decompiler` must be installed for the typecheck to succeed
+          # with `--python-version=3.8
+          python-version: "3.8"
           allow-prereleases: true
       - run: curl -LsSf https://astral.sh/uv/install.sh | sh
       - run: uv pip install -e .[dev] --system
-      - run: mypy
+      - run: mypy --python-version=${{ matrix.python-version }}
 
   flake8:
     name: flake8