pyproject: suppress superfluous warning.

.github/workflows/main.yaml

@@ -69,6 +69,8 @@ jobs:
           git fetch --tags https://github.com/amaranth-lang/amaranth.git
       - name: Set up PDM
         uses: pdm-project/setup-pdm@v4
+        with:
+          python-version: '3.12'
       - name: Install dependencies
         run: |
           pdm install --dev
@@ -88,6 +90,8 @@ jobs:
         uses: actions/checkout@v4
       - name: Set up PDM
         uses: pdm-project/setup-pdm@v4
+        with:
+          python-version: '3.12'
       - name: Install dependencies
         run: |
           pdm install --dev