Remove 'b' from sys.stdout.mode

[asottile]

Resolves #5257

[codecov]

Codecov Report

Merging #5262 into master will decrease coverage by 2.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #5262     +/-   ##
=========================================
- Coverage   93.97%   91.68%   -2.3%     
=========================================
  Files         115      115             
  Lines       26159    26163      +4     
  Branches     2578     2578             
=========================================
- Hits        24583    23987    -596     
- Misses       1259     1853    +594     
- Partials      317      323      +6

Impacted Files	Coverage Δ
testing/test_capture.py	99.25% <100%> (ø)	⬆️
src/_pytest/capture.py	95.11% <100%> (+0.02%)	⬆️
testing/test_session.py	24.05% <0%> (-75.95%)	⬇️
testing/test_runner_xunit.py	28.57% <0%> (-71.43%)	⬇️
testing/test_runner.py	34.7% <0%> (-61.61%)	⬇️
testing/test_reports.py	10.49% <0%> (-46.41%)	⬇️
testing/test_resultlog.py	53.6% <0%> (-46.4%)	⬇️
src/_pytest/reports.py	68.96% <0%> (-23.16%)	⬇️
src/_pytest/resultlog.py	67.14% <0%> (-20%)	⬇️
src/_pytest/main.py	92.1% <0%> (-3.95%)	⬇️
... and 10 more

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