Specify explicitly Ubuntu version as there is no python 3.5, 3.6 in the latest

Author: andrivet

.github/workflows/tests.yml

@@ -8,10 +8,10 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, '3.10']
+        python-version: ['2.7', '3.5', '3.6', '3.7', '3.8', '3.9', '3.10']
 
     steps:
     - uses: actions/checkout@v1