ci(test): Test the oldest supported python against the oldest and latest tmux

Author: tony

.github/workflows/tests.yml

@@ -12,6 +12,12 @@ jobs:
       matrix:
         python-version: [ '3.10' ]
         tmux-version: [ '2.6', '2.7', '2.8', '3.0a', '3.1b', '3.2a', 'master' ]
+        # balance ci coverage across supported python/tmux versions with CI speed
+        include:
+          - python-version: '3.7'
+            tmux-version: '2.6'
+          - python-version: '3.7'
+            tmux-version: 'master'
     steps:
     - uses: actions/checkout@v1