Remove support for [pytest] in setup.cfg files and pytest_plugins in non-top-level conftests

[nicoddemus]

Fix #3086, Fix #4548

[codecov]

Codecov Report

Merging #4566 into features will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #4566      +/-   ##
============================================
+ Coverage     95.73%   95.77%   +0.03%     
============================================
  Files           111      111              
  Lines         24881    24879       -2     
  Branches       2451     2451              
============================================
+ Hits          23820    23827       +7     
+ Misses          745      743       -2     
+ Partials        316      309       -7

Flag	Coverage Δ
#docs	29.41% <22.22%> (+0.08%)	⬆️
#doctesting	29.41% <22.22%> (+0.08%)	⬆️
#linting	29.41% <22.22%> (+0.08%)	⬆️
#linux	95.59% <100%> (+0.03%)	⬆️
#nobyte	92.42% <100%> (ø)	⬆️
#numpy	93.22% <100%> (+0.01%)	⬆️
#pexpect	41.89% <22.22%> (+0.03%)	⬆️
#py27	93.81% <100%> (+0.02%)	⬆️
#py34	91.9% <100%> (+0.05%)	⬆️
#py35	91.92% <100%> (+0.75%)	⬆️
#py36	91.94% <100%> (+0.05%)	⬆️
#py37	93.95% <100%> (+0.02%)	⬆️
#trial	93.22% <100%> (+0.01%)	⬆️
#windows	93.96% <100%> (ø)	⬆️
#xdist	93.81% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/config/__init__.py	93.65% <100%> (+0.01%)	⬆️
src/_pytest/config/findpaths.py	94% <100%> (-0.12%)	⬇️
testing/deprecated_test.py	100% <100%> (ø)	⬆️
src/_pytest/deprecated.py	100% <100%> (ø)	⬆️
src/_pytest/warning_types.py	92.85% <0%> (-7.15%)	⬇️
src/_pytest/capture.py	94.11% <0%> (+0.45%)	⬆️
src/_pytest/terminal.py	91.7% <0%> (+0.84%)	⬆️
src/_pytest/cacheprovider.py	97.16% <0%> (+1.41%)	⬆️

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