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

pyproject.toml

@@ -39,6 +39,10 @@ amaranth-rpc = "amaranth.rpc:main"
 requires = ["pdm-backend"]
 build-backend = "pdm.backend"
 
+[tool.pdm]
+# Remove this once we no longer support Python 3.8.
+ignore_package_warnings = ["sphinx*"]
+
 [tool.pdm.build]
 # If amaranth 0.3 is checked out with git (e.g. as a part of a persistent editable install or
 # a git worktree cached by tools like poetry), it can have an empty `nmigen` directory left over,