collect: python: fix AssertionError with broken symlinks

[blueyed]

Fixes #4782.

[blueyed]

Should we remove the asserts, but only keep them in features?
#4744 (comment)

[codecov]

Codecov Report

Merging #4784 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4784      +/-   ##
==========================================
+ Coverage   95.44%   95.66%   +0.22%     
==========================================
  Files         113      113              
  Lines       25041    25054      +13     
  Branches     2486     2488       +2     
==========================================
+ Hits        23900    23968      +68     
+ Misses        806      767      -39     
+ Partials      335      319      -16

Flag	Coverage Δ
#docs	29.61% <36.84%> (+0.16%)	⬆️
#doctesting	29.61% <36.84%> (+0.16%)	⬆️
#linting	29.61% <36.84%> (+0.16%)	⬆️
#linux	95.49% <100%> (+0.05%)	⬆️
#nobyte	92.21% <100%> (+0.63%)	⬆️
#numpy	93.02% <47.36%> (+50.95%)	⬆️
#pexpect	42.07% <15.78%> (ø)	⬆️
#pluggymaster	93.73% <100%> (+0.23%)	⬆️
#py27	93.69% <100%> (+0.14%)	⬆️
#py34	91.69% <100%> (?)
#py35	91.69% <100%> (?)
#py36	91.71% <100%> (+0.23%)	⬆️
#py37	93.8% <100%> (+0.19%)	⬆️
#trial	93.02% <47.36%> (+50.95%)	⬆️
#windows	93.7% <47.36%> (?)
#xdist	93.63% <100%> (+0.15%)	⬆️

Impacted Files	Coverage Δ
testing/test_collection.py	99.8% <100%> (ø)	⬆️
src/_pytest/python.py	92.82% <100%> (+0.02%)	⬆️
src/_pytest/main.py	95.93% <100%> (ø)	⬆️
src/_pytest/fixtures.py	97.91% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.23% <0%> (+0.3%)	⬆️
src/_pytest/pytester.py	87.44% <0%> (+0.42%)	⬆️
testing/test_pdb.py	99.07% <0%> (+0.46%)	⬆️
src/_pytest/terminal.py	91.85% <0%> (+0.81%)	⬆️
src/_pytest/nodes.py	95.52% <0%> (+0.99%)	⬆️
testing/acceptance_test.py	97.86% <0%> (+1.06%)	⬆️
... and 5 more

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[nicoddemus]

Thanks @blueyed!

About the assert, I should have been more clear: we should avoid plain asserts, because when they fail they don't add any useful information. Asserts where we pass along a failure message should be fine. 👍