use pinned ubuntu 20.04

.github/workflows/main.yml

@@ -9,20 +9,16 @@ on:
 jobs:
   build-n-test-n-coverage:
     name: Build, test and code coverage
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9, 3.10, 3.11, 3.12]
-    env:
-      OS: ubuntu-latest
-      PYTHON: "3.8"
-
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install dependencies
@@ -33,7 +29,7 @@ jobs:
       - name: Run the tests
         run: tox
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v1
+        uses: codecov/codecov-action@v4
         with:
           file: ./coverage.xml
           flags: unittests