ci: Test minimum dependencies in CI

Author: dbanty

.github/workflows/checks.yml

@@ -37,8 +37,13 @@ jobs:
         run: pip install pdm
 
       - name: Install Dependencies
+        if: matrix.python != '3.9'
         run: pdm install
 
+      - name: Install minimum dependencies
+        if: matrix.python == '3.9'
+        run: pdm install -L pdm.minimal.lock
+
       - name: Check formatting
         run: pdm run ruff format . --check