use 20.04

Author: fangq

.github/workflows/run_test.yml

@@ -4,7 +4,7 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ["2.7", "3.7", "3.8", "3.9", "3.10", "3.11"]