Restrict pytest to < 6.0

[rhodrin]

With the release of pytest 6.0.0 a previous warning seems to now be an error. See here who have also noted this issue. Latest pytest issues here.

[codecov]

Codecov Report

Merging #1411 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1411      +/-   ##
==========================================
+ Coverage   86.93%   86.98%   +0.04%     
==========================================
  Files         188      188              
  Lines       26985    26985              
  Branches     3664     3664              
==========================================
+ Hits        23459    23472      +13     
+ Misses       3098     3087      -11     
+ Partials      428      426       -2     

Impacted Files	Coverage Δ
devito/ir/iet/visitors.py	82.65% <0.00%> (+0.42%)	⬆️
devito/compiler.py	55.40% <0.00%> (+3.71%)	⬆️

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[rhodrin]

Geez, MPI tests hanging. Will investigate either a bit later or more likely in the morning now.