ci(tmux): Add tmux 3.4 to test matrix

See also: https://github.com/tmux/tmux/blob/3.4/CHANGES

Author: tony

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         python-version: ['3.12']
-        tmux-version: ['2.6', '2.7', '2.8', '3.0a', '3.1b', '3.2a', '3.3a', 'master']
+        tmux-version: ['2.6', '2.7', '2.8', '3.0a', '3.1b', '3.2a', '3.3a', '3.4', 'master']
         # balance ci coverage across supported python/tmux versions with CI speed
         include:
           - python-version: '3.8'