Warning subclasses

[nicoddemus]

Fix #5177

[codecov]

Codecov Report

Merging #5179 into features will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #5179      +/-   ##
============================================
- Coverage     96.11%   96.11%   -0.01%     
============================================
  Files           115      115              
  Lines         25980    25992      +12     
  Branches       2561     2561              
============================================
+ Hits          24971    24981      +10     
- Misses          706      707       +1     
- Partials        303      304       +1

Impacted Files	Coverage Δ
src/_pytest/assertion/rewrite.py	95.47% <100%> (ø)	⬆️
src/_pytest/config/__init__.py	94.43% <100%> (ø)	⬆️
src/pytest.py	100% <100%> (ø)	⬆️
src/_pytest/warning_types.py	100% <100%> (ø)	⬆️
src/_pytest/python.py	93.31% <100%> (ø)	⬆️
src/_pytest/junitxml.py	95.25% <100%> (ø)	⬆️
src/_pytest/cacheprovider.py	97.65% <100%> (ø)	⬆️
testing/test_warnings.py	98.9% <100%> (ø)	⬆️
src/_pytest/mark/structures.py	92.52% <100%> (ø)	⬆️
src/_pytest/compat.py	95.97% <0%> (-1.01%)	⬇️

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