collection: _recurse: skip __pycache__

[blueyed]

Taken out of #4237.
According to https://codecov.io/gh/pytest-dev/pytest/commit/5850de62f11785ade9c980a9243f07d804d1f976 it should not be covered completely: python's _recurse was copied from the Session, which might not make sense completely, or would just need some tests for the pytest_ignore_collect code therein.

[codecov]

Codecov Report

Merging #4250 into features will increase coverage by <.01%.
The diff coverage is 86.36%.

@@             Coverage Diff              @@
##           features    #4250      +/-   ##
============================================
+ Coverage     95.83%   95.83%   +<.01%     
============================================
  Files           111      111              
  Lines         24768    24777       +9     
  Branches       2414     2417       +3     
============================================
+ Hits          23737    23746       +9     
  Misses          734      734              
  Partials        297      297

Flag	Coverage Δ
#docs	28.93% <72.72%> (+0.01%)	⬆️
#doctesting	28.93% <72.72%> (+0.01%)	⬆️
#linting	28.93% <72.72%> (+0.01%)	⬆️
#linux	95.62% <86.36%> (ø)	⬆️
#nobyte	91.87% <68.18%> (-0.03%)	⬇️
#numpy	93% <81.81%> (-0.02%)	⬇️
#pexpect	41.62% <59.09%> (+0.01%)	⬆️
#py27	94.01% <72.72%> (-0.01%)	⬇️
#py34	92.23% <72.72%> (+0.05%)	⬆️
#py35	92.24% <72.72%> (+0.05%)	⬆️
#py36	93.94% <72.72%> (-0.01%)	⬇️
#py37	92.28% <72.72%> (+0.05%)	⬆️
#trial	93% <81.81%> (-0.02%)	⬇️
#windows	94% <86.36%> (-0.01%)	⬇️
#xdist	93.77% <86.36%> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/main.py	96.8% <100%> (+0.05%)	⬆️
src/_pytest/python.py	95.58% <62.5%> (+0.01%)	⬆️

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[blueyed]

This improves performance from 10.5s to 8.35s with the 1000-files-test-case.