pyupgrade @ py37

[tony]

pip install pyupgrade autoflake; \
pyupgrade --py37 tests/**/*.py docs/**/*.py tmuxp/**/*.py; \
poetry run autoflake tests/**/*.py docs/**/*.py tmuxp/**/*.py -i --ignore-init-module-imports; \
make black isort

tmux-python/libtmux#357

[codecov]

Codecov Report

Merging #745 (f35bd43) into master (27bb59c) will decrease coverage by 0.03%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
- Coverage   75.96%   75.93%   -0.04%     
==========================================
  Files           7        7              
  Lines        1190     1180      -10     
  Branches      311      311              
==========================================
- Hits          904      896       -8     
+ Misses        199      197       -2     
  Partials       87       87              

Impacted Files	Coverage Δ
tmuxp/plugin.py	100.00% <ø> (+4.65%)	⬆️
tmuxp/shell.py	40.71% <0.00%> (ø)
tmuxp/cli.py	73.46% <71.42%> (-0.06%)	⬇️
tmuxp/exc.py	94.44% <100.00%> (-1.02%)	⬇️
tmuxp/workspacebuilder.py	85.26% <100.00%> (ø)

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