Show deprecation warnings even if filters are customized

[nicoddemus]

Fix #4013

[coveralls]

Coverage increased (+0.02%) to 93.923% when pulling c301847 on nicoddemus:deprecation-warnings-4013 into 83802d1 on pytest-dev:features.

[codecov]

Codecov Report

Merging #4019 into features will increase coverage by 0.03%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##           features    #4019      +/-   ##
============================================
+ Coverage      94.5%   94.54%   +0.03%     
============================================
  Files           107      107              
  Lines         23734    23734              
  Branches       2357     2358       +1     
============================================
+ Hits          22430    22439       +9     
+ Misses          993      987       -6     
+ Partials        311      308       -3

Flag	Coverage Δ
#doctesting	28.52% <28.57%> (+0.05%)	⬆️
#linux	94.41% <85.71%> (+0.03%)	⬆️
#nobyte	0% <0%> (ø)	⬆️
#numpy	28.14% <28.57%> (ø)	⬆️
#pexpect	0% <0%> (ø)	⬆️
#pluggymaster	93.56% <85.71%> (-0.06%)	⬇️
#py27	92.66% <85.71%> (+0.03%)	⬆️
#py34	92.1% <85.71%> (-0.08%)	⬇️
#py35	92.21% <85.71%> (+0.02%)	⬆️
#py36	92.78% <85.71%> (+0.03%)	⬆️
#py37	92.27% <85.71%> (-0.12%)	⬇️
#trial	31.03% <28.57%> (-0.17%)	⬇️
#windows	93.07% <85.71%> (-0.78%)	⬇️
#xdist	18.43% <28.57%> (-0.04%)	⬇️

Impacted Files	Coverage Δ
testing/test_warnings.py	98.49% <100%> (+0.02%)	⬆️
src/_pytest/warnings.py	87.01% <66.66%> (-1.6%)	⬇️
src/_pytest/terminal.py	91.6% <0%> (+1.74%)	⬆️

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[jaraco]

Looks great. Thanks!