Import tweaks

[tony]

Cherry-picked from #383 Import improvements

[codecov]

Codecov Report

Merging #384 (51ace8f) into master (70d99a7) will increase coverage by 0.00%.
The diff coverage is 93.33%.

@@           Coverage Diff           @@
##           master     #384   +/-   ##
=======================================
  Coverage   88.35%   88.36%           
=======================================
  Files          16       16           
  Lines        1572     1581    +9     
=======================================
+ Hits         1389     1397    +8     
- Misses        183      184    +1     

Impacted Files	Coverage Δ
libtmux/session.py	80.95% <66.66%> (-0.49%)	⬇️
tests/test_server.py	100.00% <100.00%> (ø)
tests/test_session.py	98.55% <100.00%> (+0.03%)	⬆️
tests/test_tmuxobject.py	100.00% <100.00%> (ø)
tests/test_window.py	98.78% <100.00%> (+0.02%)	⬆️

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