ci: drop Python 3.9 from test matrix

meshcore >=2.2.31 requires Python >=3.10; 3.9 is not installable
and was failing the dependency install step in CI.

.github/workflows/test.yml

@@ -98,7 +98,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.11", "3.12"]
+        python-version: ["3.11", "3.12"]
     steps:
       - uses: actions/checkout@v4