CI: Pin to python 3.12

.github/workflows/ci.yml

@@ -9,12 +9,19 @@ on:
       - .gitignore
       - "*.md"
 
+env:
+  PYTHON_VERSION: '3.12'
+
 jobs:
   pytest:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
 
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
       - name: "Install dependencies"
         run: pip install -r requirements.txt && pip install -r test/requirements.txt
 
@@ -26,6 +33,10 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
       - name: "Install dependencies"
         run: |
           pip install -r requirements.txt