Update testing.yml

drop support for Python 3.9, add support for Python 3.12 following materialsproject/pymatgen#4009

Author: matthewkuner

.github/workflows/testing.yml

@@ -9,7 +9,7 @@ jobs:
       max-parallel: 20
       matrix:
         os: [ubuntu-latest, windows-latest]
-        python-version: ["3.9", "3.10", "3.11"]
+        python-version: ["3.10", "3.11", 3.12]
 
     runs-on: ${{ matrix.os }}