ci(tmux): Add tmux 3.4 to test matrix

[tony]

See also:

• https://github.com/tmux/tmux/blob/3.4/CHANGES
• https://github.com/tmux-python/libtmux/pull/524

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b35f743) 70.38% compared to head (87e32e1) 70.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #909 +/- ## ========================================== - Coverage 70.38% 70.33% -0.06% ========================================== Files 26 26 Lines 1955 1955 Branches 409 400 -9 ========================================== - Hits 1376 1375 -1 - Misses 453 455 +2 + Partials 126 125 -1 ```

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