Fixed CI by pinning runner version

fixes pulp#589

.github/workflows/main.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Lint code
         run: make lint
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs:
       - lint
     strategy:
@@ -77,19 +77,14 @@ jobs:
         run: .ci/run_container.sh make test
   ready-to-ship:
     runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-          - "3.10"
     steps:
       - uses: actions/checkout@v3
       - run: |
           git fetch --prune --unshallow
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: ${{ matrix.python }}
+          python-version: "3.10"
       - name: Install python dependencies
         run: |
           pip install pygithub

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ env:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:

CHANGES/589.misc

@@ -0,0 +1 @@
+Switched CI runner to ubuntu-20.04 for older versions of python.