Python 3 modernization

[tony]

No description provided.

[codecov]

Codecov Report

Merging #314 (f42469d) into master (73d2bc5) will decrease coverage by 0.20%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   87.20%   87.00%   -0.21%     
==========================================
  Files          16       16              
  Lines        1540     1516      -24     
==========================================
- Hits         1343     1319      -24     
  Misses        197      197              

Impacted Files	Coverage Δ
libtmux/common.py	86.43% <ø> (ø)
libtmux/exc.py	100.00% <ø> (ø)
libtmux/formats.py	100.00% <ø> (ø)
libtmux/pane.py	82.35% <ø> (-0.26%)	⬇️
libtmux/session.py	80.95% <ø> (-0.23%)	⬇️
libtmux/test.py	42.64% <ø> (-1.64%)	⬇️
libtmux/window.py	83.23% <ø> (-0.20%)	⬇️
tests/conftest.py	84.78% <ø> (ø)
tests/test_common.py	100.00% <ø> (ø)
tests/test_pane.py	100.00% <ø> (ø)
... and 5 more

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