pytest: Fix warning regarding usefixtures

[tony]

Changes

pytest: fix warning

conftest.py:52
  libtmux/conftest.py:52: PytestRemovedIn9Warning: Marks applied to fixtures have no effect
  See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
    def setup(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (498405f) 89.06% compared to head (0b3624c) 89.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
+ Coverage   89.06%   89.15%   +0.09%     
==========================================
  Files          36       36              
  Lines        3557     3559       +2     
  Branches      502      501       -1     
==========================================
+ Hits         3168     3173       +5     
+ Misses        268      265       -3     
  Partials      121      121              

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