RF: do not require testing harnesses for regular usage

[yarikoptic]

continuation to #2850 , I guess awaiting on verdict from #2853 as well

[yarikoptic]

additional note/motivation is that pytest >= 3.6 cannot even be satisfied on stable Debian/Ubuntus (e.g. debian stretch/stable has 3.0.6).

[yarikoptic]

@oesteban do you remember "why?" (ref: a4dfe7e):

commit a4dfe7e6a2ec78ba9cea20005799001f16ddcb99 (origin/oesteban-patch-2)
Author: Oscar Esteban <code@oscaresteban.es>
Date:   Thu Dec 13 16:41:55 2018 -0800

    [FIX] Update pytest req'd version to 3.6
    
    Fixes #2826.

diff --git a/nipype/info.py b/nipype/info.py
index 1cf361c40..be6edb713 100644
--- a/nipype/info.py
+++ b/nipype/info.py
@@ -108,7 +108,7 @@ NUMPY_MIN_VERSION_37 = '1.15.3'
 SCIPY_MIN_VERSION = '0.14'
 TRAITS_MIN_VERSION = '4.6'
 DATEUTIL_MIN_VERSION = '2.2'
-PYTEST_MIN_VERSION = '3.0'
+PYTEST_MIN_VERSION = '3.6'
 FUTURE_MIN_VERSION = '0.16.0'
 SIMPLEJSON_MIN_VERSION = '3.8.0'
 PROV_VERSION = '1.5.2'

seems to run tests just fine on debian stretch with 3.0.6-1 .

[yarikoptic]

ah, @oesteban - please disregard my above questsion -- did not spot #2826 which somewhat explains the reason. I think for the Debian package I would just eliminate any versioning of the dependency since it would install most recent available, and fail if fails to build/test

[effigies]

Okay. So we may wish to drop that requirement, but that means either setting a max version on xdist, which itself set a minimum version of 3.6 (see #2826, #2827) or dropping the requirement for xdist, which will in turn require removing the -n auto from the pytest.ini. At which point, we'd want to instruct Travis and Circle to install pytest-xdist separately, and add -n auto for the sake of speed.

I'm fine with all of this. It's just a little rabbit hole.

[yarikoptic]

Thank you @effigies ! whatever you feel works ok for you is ok with me -- at least I can workaround.

[codecov-io]

Codecov Report

Merging #2854 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2854   +/-   ##
=======================================
  Coverage   67.46%   67.46%           
=======================================
  Files         341      341           
  Lines       43392    43392           
  Branches     5383     5383           
=======================================
  Hits        29276    29276           
  Misses      13419    13419           
  Partials      697      697

Flag	Coverage Δ
#smoketests	50.53% <ø> (ø)	⬆️
#unittests	64.88% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/info.py	94.2% <ø> (ø)	⬆️

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