DM-39857: Stop using pytest-flake8

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[andy-slac]

👍

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (bc92477) 87.90% compared to head (9a6506a) 87.90%.

Additional details and impacted files

@@            Coverage Diff            @@
##             main     #861     +/-   ##
=========================================
  Coverage   87.90%   87.90%             
=========================================
  Files         270      270             
  Lines       35589    35589             
  Branches     6049     7452   +1403     
=========================================
+ Hits        31285    31286      +1     
  Misses       3145     3145             
+ Partials     1159     1158      -1     

see 1 file with indirect coverage changes

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