Update TMUX_MAX_VERSION, add 3.3a to CI

[tony]

• CI: Add tmux 3.3a to test suite
• Set TMUX_MAX_VERSION: 2.4 -> 3.3

[codecov]

Codecov Report

Merging #387 (1697491) into master (973ddab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 1697491 differs from pull request most recent head 9e115cc. Consider uploading reports for the commit 9e115cc to get more accurate results

@@           Coverage Diff           @@
##           master     #387   +/-   ##
=======================================
  Coverage   88.36%   88.36%           
=======================================
  Files          16       16           
  Lines        1581     1582    +1     
=======================================
+ Hits         1397     1398    +1     
  Misses        184      184           

Impacted Files	Coverage Δ
libtmux/common.py	87.25% <100.00%> (ø)
tests/test_common.py	98.27% <100.00%> (+0.01%)	⬆️

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