CI: add Python 3.13 and 3.14 to test matrix (#360)

The matrix previously topped out at 3.12, leaving no coverage for 3.13
or 3.14. Add both so the suite validates newer interpreters — in
particular the Python 3.14 argparse change (nested argument groups are
now a hard error) reported in #318/#359, which was fixed in v2.6.2.

Claude-Session: https://claude.ai/code/session_018iM7tTGPFS6ija49yAQpSM

Author: iskandr

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
 
     steps:
       - name: Checkout repository