ci: use python 3.11 instead of 3.x

We picked up 3.12 for which there aren't binary wheels yet for
all dependencies.

Author: maartenbreddels

.github/workflows/unit.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: "3.x"
+          python-version: "3.11"
       - name: Install dependencies
         run: pip install -r .github/requirements.txt
       - name: Run black
@@ -35,7 +35,7 @@ jobs:
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: "3.x"
+          python-version: "3.11"
 
       - name: Install dependencies
         run: |