fixed workflow python version

.gitea/workflows/tests.yaml

@@ -4,14 +4,14 @@ on: [push]
 
 jobs:
   unittests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Check out repository code
         uses: actions/checkout@v4
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: '3.x'
+          python-version: '3.13'
       - name: Test with unittest
         run: |
           pip install unittest