ci: Add libevent-2.1-7

Author: tony

.github/workflows/tests.yml

@@ -65,6 +65,7 @@ jobs:
       - name: Test with pytest
         continue-on-error: ${{ matrix.tmux-version == 'master' }}
         run: |
+          sudo apt install libevent-2.1-7
           export PATH=$HOME/tmux-builds/tmux-${{ matrix.tmux-version }}/bin:$PATH
           ls $HOME/tmux-builds/tmux-${{ matrix.tmux-version }}/bin
           tmux -V