Fix access denied error when deleting a stale temporary directory

[nicoddemus]

Fix #4262

[codecov]

Codecov Report

Merging #4267 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4267      +/-   ##
==========================================
+ Coverage   95.87%   95.87%   +<.01%     
==========================================
  Files         109      109              
  Lines       24608    24626      +18     
  Branches     2394     2395       +1     
==========================================
+ Hits        23593    23611      +18     
  Misses        720      720              
  Partials      295      295

Flag	Coverage Δ
#docs	29.01% <0%> (-0.03%)	⬇️
#doctesting	29.01% <0%> (-0.03%)	⬇️
#linting	29.01% <0%> (-0.03%)	⬇️
#linux	95.66% <100%> (ø)	⬆️
#nobyte	91.97% <100%> (-0.06%)	⬇️
#numpy	43.37% <0%> (-49.66%)	⬇️
#pexpect	41.62% <0%> (-0.04%)	⬇️
#py27	94.05% <100%> (ø)	⬆️
#py34	92.22% <100%> (-0.09%)	⬇️
#py35	92.36% <100%> (+0.03%)	⬆️
#py36	94% <100%> (ø)	⬆️
#py37	92.37% <100%> (+0.03%)	⬆️
#trial	43.37% <0%> (-49.66%)	⬇️
#windows	94.11% <100%> (-0.03%)	⬇️
#xdist	93.9% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
testing/test_pathlib.py	100% <100%> (ø)
src/_pytest/pathlib.py	89.61% <100%> (+0.35%)	⬆️

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