ci: use ubuntu-20.04 instead of latest temporary

.github/workflows/testing_initiative.yml

@@ -32,14 +32,17 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os:
+         - ubuntu-20.04
+         - macos-latest
+         - windows-latest
         python-version: [2.7, 3.6, 3.7, 3.8, 3.9]
         no-coverage: [0]
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             python-version: pypy-2.7
             no-coverage: 1
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             python-version: pypy-3.6
             no-coverage: 1
     steps:
@@ -71,7 +74,7 @@ jobs:
   coveralls:
     name: Finish Coveralls
     needs: test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     container: python:3-slim
     steps:
     - name: Install coveralls
@@ -87,7 +90,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os:
+          - ubuntu-20.04
         python-version: [3.6]
     steps:
       - uses: actions/checkout@v2