[4.6] Fix warnings with attrs 19.2 and fix object assertions (#5902)

[mimi1vx]

No description provided.

[codecov]

Codecov Report

Merging #5944 into 4.6-maintenance will increase coverage by 0.27%.
The diff coverage is 83.33%.

@@                Coverage Diff                 @@
##           4.6-maintenance   #5944      +/-   ##
==================================================
+ Coverage            95.92%   96.2%   +0.27%     
==================================================
  Files                  115     115              
  Lines                26553   26560       +7     
  Branches              2615    2616       +1     
==================================================
+ Hits                 25472   25551      +79     
+ Misses                 755     703      -52     
+ Partials               326     306      -20

Impacted Files	Coverage Δ
testing/test_assertion.py	97.67% <100%> (ø)	⬆️
src/_pytest/assertion/util.py	98.16% <100%> (ø)	⬆️
src/_pytest/mark/structures.py	92.77% <100%> (+0.04%)	⬆️
src/_pytest/compat.py	96.05% <66.66%> (+2.08%)	⬆️
src/_pytest/fixtures.py	97.55% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.25% <0%> (+0.29%)	⬆️
testing/test_terminal.py	99.84% <0%> (+0.31%)	⬆️
testing/test_config.py	99.83% <0%> (+0.32%)	⬆️
testing/test_pdb.py	99.22% <0%> (+0.38%)	⬆️
src/_pytest/pytester.py	91.66% <0%> (+0.42%)	⬆️
... and 15 more

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[nicoddemus]

Thanks a lot @mimi1vx!