ci(tests) Drop Python 3.9

See also:
- https://devguide.python.org/versions/#:~:text=Release%20manager-,3.9,-PEP%20596
- https://peps.python.org/pep-0596/

Author: tony

.github/workflows/tests.yml

@@ -12,12 +12,6 @@ jobs:
       matrix:
         python-version: ['3.14']
         tmux-version: ['2.6', '2.7', '2.8', '3.0a', '3.1b', '3.2a', '3.3a', '3.4', '3.5', 'master']
-        # balance ci coverage across supported python/tmux versions with CI speed
-        include:
-          - python-version: '3.9'
-            tmux-version: '2.6'
-          - python-version: '3.9'
-            tmux-version: 'master'
     steps:
       - uses: actions/checkout@v4