Improve comparison of byte strings (supersedes #5267)

[nicoddemus]

Supersedes #5267
Fixes #5260

[asottile]

[codecov]

Codecov Report

Merging #5495 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5495      +/-   ##
=========================================
+ Coverage   96.07%   96.1%   +0.02%     
=========================================
  Files         117     117              
  Lines       25550   25701     +151     
  Branches     2473    2536      +63     
=========================================
+ Hits        24548   24699     +151     
+ Misses        698     697       -1     
- Partials      304     305       +1

Impacted Files	Coverage Δ
testing/test_assertion.py	97.56% <100%> (+0.02%)	⬆️
src/_pytest/assertion/util.py	95.01% <100%> (+0.15%)	⬆️
src/_pytest/assertion/rewrite.py	95.98% <0%> (+1.13%)	⬆️

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[nicoddemus]

I had explicitly added @blueyed as reviewer because he wrote the original PR 😬 ... Oh well 😁

[asottile]

/o\ oops -- my bad

[blueyed]

@nicoddemus
Thanks for having picked it up, and improved it!