ci: trim the unit-test matrix to 3.9, 3.12, 3.13

Five parallel jobs added little signal: 3.9 covers the supported floor
(tomli backport, oldest dependency resolution), 3.12 is the dominant
download share (~47%), and 3.13 is the newest stable. 3.10/3.11 behave
identically for this codebase.

Author: agentscope-ai-java

.github/workflows/unit_test.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        python-version: [ '3.9', '3.10', '3.11', '3.12', '3.13' ]
+        python-version: [ '3.9', '3.12', '3.13' ]
 
 
     steps: