enable pytest 5.x and pytest-xdist >= 1.28

[mr-c]

No description provided.

[codecov]

Codecov Report

Merging #1311 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1311      +/-   ##
==========================================
+ Coverage   77.46%   77.54%   +0.08%     
==========================================
  Files          40       40              
  Lines        7451     7451              
  Branches     1912     1912              
==========================================
+ Hits         5772     5778       +6     
+ Misses       1183     1178       -5     
+ Partials      496      495       -1     

Impacted Files	Coverage Δ
cwltool/job.py	69.68% <0.00%> (+0.42%)	⬆️
cwltool/executors.py	81.04% <0.00%> (+1.89%)	⬆️

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