swtich to python 3.8.11

Author: xwings

.github/workflows/build-ci.yml

@@ -16,7 +16,7 @@ jobs:
           - os: ubuntu-20.04
             python-version: 3.7.6       
           - os: ubuntu-20.04
-            python-version: 3.9.5
+            python-version: 3.8.11
           - os: windows-2019
             python-version: 3.7.6                 
           # - os: macos-10.15