ci: update few workflows to use Python 3.12

The HTML doc build workflow and PR assigner recently started failing to
install python packages, breaking all backports. Seems to work when
using Python 3.12, this is already done in main in bacb99d.

Signed-off-by: Fabio Baltieri <[email protected]>

Author: fabiobaltieri

.github/workflows/assigner.yml

@@ -22,6 +22,11 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: 3.12
+
     - name: Install Python dependencies
       run: |
         sudo pip3 install -U setuptools wheel pip

.github/workflows/doc-build.yml

@@ -70,6 +70,11 @@ jobs:
       cancel-in-progress: true
 
     steps:
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: 3.12
+
     - name: install-pkgs
       run: |
         sudo apt-get update
@@ -202,6 +207,11 @@ jobs:
     - name: checkout
       uses: actions/checkout@v4
 
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: 3.12
+
     - name: install-pkgs
       run: |
         apt-get update