Fix Github Actions

Author: dolfinus

.github/workflows/test.yml

@@ -5,7 +5,7 @@ on: push
 jobs:
   tests:
     name: Run tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         python-version: ['2.7', '3.3', '3.4', '3.5', '3.6', '3.7', '3.8', '3.9']