CI: use python 3.11 for the nightly tests (#582)

Author: twoertwein

.github/workflows/optional.yml

@@ -18,7 +18,7 @@ jobs:
       uses: ./.github/setup
       with:
         os: ubuntu-latest
-        python-version: '3.10'
+        python-version: '3.11'
 
     - name: Run pytest (against pandas nightly)
       run: poetry run poe pytest --nightly
@@ -34,7 +34,7 @@ jobs:
       uses: ./.github/setup
       with:
         os: ubuntu-latest
-        python-version: '3.10'
+        python-version: '3.11'
 
     - name: Run mypy tests with mypy nightly
       run: poetry run poe mypy --mypy_nightly