ci(test): use ubuntu 20.04 version to run tests

.github/workflows/test.yml

@@ -5,21 +5,21 @@ on:
     branches:
       - master
     paths:
-      - '.github/workflows/test.yml'
+      - ".github/workflows/test.yml"
-      - '**.py'
+      - "**.py"
   pull_request:
     branches:
       - master
     paths:
-      - '.github/workflows/test.yml'
+      - ".github/workflows/test.yml"
-      - '**.py'
+      - "**.py"
 
 jobs:
   test:
     strategy:
       matrix:
-        python: ['3.6', '3.7', '3.8', '3.9']
+        python: ["3.6", "3.7", "3.8", "3.9"]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2