stick to ubuntu 20.04 for test CI workflow, so we can keep testing wi…

…th Python 3.6

.github/workflows/tests.yaml

@@ -2,17 +2,17 @@ name: Run tests
 on: [push, pull_request]
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python: [3.6, 3.7, 3.8, 3.9, '3.10']
       fail-fast: false
     steps:
       - name: Check out repository code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup Python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v5
         with:
           python-version: ${{matrix.python}}