CI: Install unreleased "py" package for 3.11-dev

Author: tk0miya

.github/workflows/main.yml

@@ -44,6 +44,9 @@ jobs:
       if: endsWith(matrix.python, '-dev')
       with:
         python-version: ${{ matrix.python }}
+    - name: "Set up unreleased py package (refs: https://github.com/pytest-dev/py/issues/273)"
+      run: pip install git+https://github.com/pytest-dev/py
+      if: endsWith(matrix.python, '-dev')
     - name: Check Python version
       run: python --version
     - name: Install graphviz