ci: fix pytest-xdist deps for older pytest, test pytest 4.6

[blueyed]

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[codecov-io]

Codecov Report

Merging #177 into master will increase coverage by 0.83%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   85.38%   86.22%   +0.83%     
==========================================
  Files           2        2              
  Lines         479      479              
  Branches       85       85              
==========================================
+ Hits          409      413       +4     
+ Misses         45       41       -4     
  Partials       25       25

Impacted Files	Coverage Δ
pytest_sugar.py	82.16% <0%> (+1.16%)	⬆️

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