fix #4386 - restructure construction and partial state of ExceptionInfo

[RonnyPfannschmidt]

fixes #4386

[codecov]

Codecov Report

Merging #4438 into features will increase coverage by <.01%.
The diff coverage is 95.83%.

@@             Coverage Diff              @@
##           features    #4438      +/-   ##
============================================
+ Coverage     95.87%   95.87%   +<.01%     
============================================
  Files           111      111              
  Lines         25024    25044      +20     
  Branches       2444     2447       +3     
============================================
+ Hits          23992    24012      +20     
  Misses          730      730              
  Partials        302      302

Flag	Coverage Δ
#docs	30.2% <61.81%> (+0.12%)	⬆️
#doctesting	30.2% <61.81%> (+0.12%)	⬆️
#linting	30.2% <61.81%> (+0.12%)	⬆️
#linux	95.71% <95.83%> (ø)	⬆️
#nobyte	92.63% <95.83%> (ø)	⬆️
#numpy	93.43% <95.83%> (ø)	⬆️
#pexpect	41.95% <74.54%> (+0.04%)	⬆️
#py27	94.01% <95.83%> (ø)	⬆️
#py34	92.15% <95.83%> (+0.07%)	⬆️
#py35	92.17% <95.83%> (+0.07%)	⬆️
#py36	92.19% <95.83%> (+0.07%)	⬆️
#py37	94.08% <95.83%> (+0.02%)	⬆️
#trial	93.43% <95.83%> (ø)	⬆️
#windows	94.09% <95.83%> (ø)	⬆️
#xdist	93.94% <95.83%> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/assertion/util.py	98.11% <ø> (ø)	⬆️
src/_pytest/unittest.py	94.41% <100%> (+0.06%)	⬆️
src/_pytest/runner.py	96.91% <100%> (ø)	⬆️
src/_pytest/main.py	96.96% <100%> (ø)	⬆️
src/_pytest/python_api.py	97.42% <100%> (ø)	⬆️
src/_pytest/python.py	95.27% <100%> (ø)	⬆️
testing/code/test_code.py	94.2% <100%> (ø)	⬆️
testing/test_resultlog.py	100% <100%> (ø)	⬆️
testing/code/test_excinfo.py	96.78% <100%> (+0.01%)	⬆️
testing/test_runner.py	96.91% <75%> (-0.03%)	⬇️
... and 1 more

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[nicoddemus]

Nice work, I like the approach! Just some minor corrections/docs and suggestions. 👍

[nicoddemus]

Great work!

[blueyed]

This breaks xdist:

        except:  # noqa
            from _pytest._code import ExceptionInfo
    
>           excinfo = ExceptionInfo()
E           TypeError: __init__() missing 1 required positional argument: 'excinfo'

…/Vcs/pytest-xdist/xdist/workermanage.py:350: TypeError

[RonnyPfannschmidt]

pytest-dev/pytest-xdist#387