Modernize Python syntax using pyupgrade --py36-plus

[mayeut]

There were some things left from previous manual updates, use of pyupgrade --py36-plus found those:

• Do not specify mode=rt for open as it is the default
• Use OSError instead of IOError
• Modernize class syntax
• Convert some string formatting in f-strings

[codecov]

Codecov Report

Merging #278 (5bf3282) into master (7ca5a84) will not change coverage.
The diff coverage is 63.63%.

@@           Coverage Diff           @@
##           master     #278   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files          20       20           
  Lines        1043     1043           
  Branches      223      223           
=======================================
  Hits          948      948           
  Misses         54       54           
  Partials       41       41           

Impacted Files	Coverage Δ
auditwheel/lddtree.py	92.90% <0.00%> (ø)
auditwheel/main_show.py	70.58% <40.00%> (ø)
auditwheel/condatools.py	100.00% <100.00%> (ø)
auditwheel/repair.py	85.59% <100.00%> (ø)
auditwheel/tmpdirs.py	100.00% <100.00%> (ø)
auditwheel/wheeltools.py	92.00% <100.00%> (ø)

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