Fix handling of collect_ignore from parent conftest

[blueyed]

_collectfile should be called on files only.

Fixes #4592.

TODO:

• squash
• [ ] remove assertions?

[blueyed]

I assume this fixes other similar issues.

[nicoddemus]

Great work @blueyed!

[codecov]

Codecov Report

Merging #4744 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4744      +/-   ##
==========================================
+ Coverage   95.67%   95.67%   +<.01%     
==========================================
  Files         113      113              
  Lines       24973    24984      +11     
  Branches     2480     2481       +1     
==========================================
+ Hits        23892    23903      +11     
- Misses        762      764       +2     
+ Partials      319      317       -2

Flag	Coverage Δ
#docs	29.57% <46.66%> (-0.01%)	⬇️
#doctesting	29.57% <46.66%> (-0.01%)	⬇️
#linting	29.57% <46.66%> (-0.01%)	⬇️
#linux	95.5% <100%> (ø)	⬆️
#nobyte	91.66% <100%> (-0.62%)	⬇️
#numpy	42.06% <26.66%> (-51.03%)	⬇️
#pexpect	42.06% <26.66%> (-0.02%)	⬇️
#py27	93.69% <100%> (+0.03%)	⬆️
#py34	91.65% <100%> (-0.05%)	⬇️
#py35	91.65% <100%> (-0.05%)	⬇️
#py36	91.63% <100%> (-0.11%)	⬇️
#py37	93.77% <100%> (-0.04%)	⬇️
#trial	42.06% <26.66%> (-51.03%)	⬇️
#windows	93.16% <100%> (-0.7%)	⬇️
#xdist	93.72% <100%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/python.py	92.67% <100%> (+0.26%)	⬆️
src/_pytest/main.py	95.93% <100%> (ø)	⬆️
testing/test_collection.py	99.79% <100%> (ø)	⬆️
src/_pytest/capture.py	93.66% <0%> (-0.46%)	⬇️
src/_pytest/pytester.py	87% <0%> (-0.43%)	⬇️
src/_pytest/cacheprovider.py	97.16% <0%> (+1.41%)	⬆️

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[blueyed]

Build failure appears unrelated: https://travis-ci.org/pytest-dev/pytest/jobs/490660838 (pyinstaller)

[RonnyPfannschmidt]

build failure is a known pip issue, based on context we can merge, great work

[blueyed]

See #4751 for fixing the CI issue.

[blueyed]

In retrospect the asserts should have not been added to master (the bugfix release), but only to features.
(causing #4782)