Revert "DEBUG: Rollback to Python 3.9"

This reverts commit fecc4c2.

Author: tkf

.github/workflows/system.yml

@@ -18,7 +18,7 @@ jobs:
           - macos-latest
           - windows-latest
         architecture: [x64]
-        python-version: ['3.9']
+        python-version: ['3.x']
         julia-version:
           - '1.0'
           - '1'