Turn PytestDeprecationWarning into error

[nicoddemus]

Fix #5402

[codecov]

Codecov Report

Merging #5410 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5410      +/-   ##
==========================================
+ Coverage   95.98%   95.98%   +<.01%     
==========================================
  Files         114      114              
  Lines       25496    25507      +11     
  Branches     2474     2476       +2     
==========================================
+ Hits        24473    24484      +11     
  Misses        718      718              
  Partials      305      305

Impacted Files	Coverage Δ
testing/test_warnings.py	100% <100%> (ø)	⬆️
src/_pytest/warnings.py	86.95% <100%> (+0.19%)	⬆️
testing/python/fixtures.py	99.09% <100%> (ø)	⬆️

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[asottile]