Fix crash on calling Server.attached_sessions

[Timoses]

fixes #180

[tony]

@Timoses Can you rebase this and #342 so tests run? Thank you!

[Timoses]

@tony : Done ; )

[tony]

@Timoses Thank you!

[codecov]

Codecov Report

Merging #341 (7198957) into master (714261b) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #341   +/-   ##
=======================================
  Coverage   87.06%   87.06%           
=======================================
  Files          16       16           
  Lines        1515     1515           
=======================================
  Hits         1319     1319           
  Misses        196      196           

Impacted Files	Coverage Δ
libtmux/server.py	79.19% <0.00%> (ø)

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[tony]

Merged via #342 @ 7198957,

Thank you @Timoses!