[CI] Fix "mypy: Check stubs" when using specific Python version (python#13975)

Author: srittau

.github/workflows/tests.yml

@@ -70,7 +70,11 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - run: curl -LsSf https://astral.sh/uv/install.sh | sh
       - run: uv pip install -r requirements-tests.txt --system
-      - run: python ./tests/mypy_test.py --platform=${{ matrix.platform }} --python-version=${{ matrix.python-version }}
+      - run: |
+          # python-version can sometimes be pinned to a specific version or to "-dev", but
+          # mypy understands only X.Y version numbers.
+          MYPY_PY_VERSION=$(echo ${{ matrix.python-version }} | cut -d - -f 1 | cut -d . -f 1-2)
+          python ./tests/mypy_test.py --platform=${{ matrix.platform }} --python-version=${MYPY_PY_VERSION}
 
   regression-tests:
     name: "mypy: Run test cases"