build(deps): libtmux 0.20.0 -> 0.21.0 to avoid isolated crashes

[tony]

This adjusts the default separator for formats to be a pipe (somewhat common in the terminal) to a rare character - ␞. This will avoid crashes with various libtmux lookups.

See also:

• FORMAT_SEPARATOR: Change to a character less likely to collide with buffers libtmux#475
• https://github.com/tmux-python/libtmux/releases/tag/v0.21.0

[codecov]

Codecov Report

Merging #865 (9e6eabe) into master (f35c39e) will not change coverage.
The diff coverage is n/a.

❗ Current head 9e6eabe differs from pull request most recent head 17d2f99. Consider uploading reports for the commit 17d2f99 to get more accurate results

@@           Coverage Diff           @@
##           master     #865   +/-   ##
=======================================
  Coverage   71.39%   71.39%           
=======================================
  Files          25       25           
  Lines        1846     1846           
  Branches      408      408           
=======================================
  Hits         1318     1318           
  Misses        409      409           
  Partials      119      119           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more