Set python default version to 3.9

Author: guparan

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
       matrix:
         os: [ubuntu-20.04, macos-10.15, windows-2019]
         sofa_branch: [master]
-        python_version: ['3.8']
+        python_version: ['3.9']
 
     steps:
       - name: Setup SOFA and environment