Remove Python 2.7

[tony]

Counterpart to tmux-python/libtmux#306

[codecov]

Codecov Report

Merging #661 (62dc1b8) into master (d611a2a) will decrease coverage by 0.26%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
- Coverage   75.78%   75.51%   -0.27%     
==========================================
  Files           8        8              
  Lines        1181     1168      -13     
  Branches      295      295              
==========================================
- Hits          895      882      -13     
  Misses        205      205              
  Partials       81       81              

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
tmuxp/exc.py	95.45% <ø> (-0.38%)	⬇️
tmuxp/shell.py	40.71% <ø> (-0.84%)	⬇️
tmuxp/util.py	88.88% <ø> (-0.31%)	⬇️
tmuxp/cli.py	73.23% <50.00%> (-0.12%)	⬇️
tmuxp/config.py	85.77% <66.66%> (-0.18%)	⬇️
tmuxp/workspacebuilder.py	87.70% <100.00%> (-0.14%)	⬇️

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