Issue a warning when a fixture named 'request' is collected

[nicoddemus]

Fix #611

[codecov]

Codecov Report

Merging #4209 into features will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           features   #4209      +/-   ##
===========================================
+ Coverage      95.8%   95.8%   +<.01%     
===========================================
  Files           109     109              
  Lines         24231   24240       +9     
  Branches       2389    2390       +1     
===========================================
+ Hits          23215   23224       +9     
  Misses          721     721              
  Partials        295     295

Flag	Coverage Δ
#docs	28.49% <36.36%> (-0.01%)	⬇️
#doctesting	28.49% <36.36%> (-0.01%)	⬇️
#linting	28.49% <36.36%> (-0.01%)	⬇️
#linux	95.58% <100%> (ø)	⬆️
#nobyte	91.87% <100%> (ø)	⬆️
#numpy	92.91% <100%> (ø)	⬆️
#pexpect	41.31% <36.36%> (-0.01%)	⬇️
#py27	93.94% <100%> (ø)	⬆️
#py34	92.27% <100%> (+0.03%)	⬆️
#py35	92.28% <100%> (+0.03%)	⬆️
#py36	93.94% <100%> (ø)	⬆️
#py37	92.31% <100%> (+0.03%)	⬆️
#trial	92.91% <100%> (ø)	⬆️
#windows	94.04% <100%> (ø)	⬆️
#xdist	93.77% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
testing/deprecated_test.py	100% <100%> (ø)	⬆️
src/_pytest/fixtures.py	97.31% <100%> (+0.01%)	⬆️
src/_pytest/deprecated.py	100% <100%> (ø)	⬆️

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