Clean up u' prefixes and py2 bytes conversions

[asottile]

No description provided.

[codecov]

Codecov Report

Merging #5400 into master will increase coverage by 3.13%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #5400      +/-   ##
==========================================
+ Coverage   89.58%   92.72%   +3.13%     
==========================================
  Files         114      114              
  Lines       25515    25827     +312     
  Branches     2480     2573      +93     
==========================================
+ Hits        22857    23947    +1090     
+ Misses       2325     1548     -777     
+ Partials      333      332       -1

Impacted Files	Coverage Δ
testing/test_terminal.py	99.53% <ø> (ø)	⬆️
testing/test_warnings.py	58.53% <ø> (+32.92%)	⬆️
testing/test_doctest.py	100% <ø> (ø)	⬆️
testing/test_junitxml.py	98.11% <ø> (ø)	⬆️
src/_pytest/compat.py	97.01% <ø> (ø)	⬆️
testing/test_capture.py	99.21% <ø> (ø)	⬆️
testing/test_nose.py	98.87% <ø> (ø)	⬆️
testing/test_assertion.py	97.53% <ø> (ø)	⬆️
testing/python/collect.py	66.87% <0%> (-22.09%)	⬇️
src/_pytest/assertion/rewrite.py	93.49% <100%> (+0.64%)	⬆️
... and 31 more

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