Remove obsolete version requirement for pytest

[c24t]

Quick update to #293 now that pytest-dev/pytest#6194 is fixed.

Note that there wouldn't be any harm in leaving the required version as is: users should pick up the latest version (5.3.1 as of this writing) which include the fix in either case.

[codecov-io]

Codecov Report

Merging #317 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #317   +/-   ##
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  Coverage   86.05%   86.05%           
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  Files          33       33           
  Lines        1628     1628           
  Branches      182      182           
=======================================
  Hits         1401     1401           
  Misses        181      181           
  Partials       46       46

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[codeboten]

LGTM

[Oberon00]

As you already said:

Note that there wouldn't be any harm in leaving the required version as is

so I would prefer if we reworded the PR title, e.g. to "Remove obsolete version requirement for pytest", since we are already using latest.