More CI Cleanup

[blink1073]

• Adopt more flake8 rules, modeled after what the pip repo uses
• Enforce a minimum code coverage

[codecov-commenter]

Codecov Report

Merging #742 (a9ce005) into main (7f2863f) will increase coverage by 0.07%.
The diff coverage is 39.47%.

@@            Coverage Diff             @@
##             main     #742      +/-   ##
==========================================
+ Coverage   70.56%   70.64%   +0.07%     
==========================================
  Files          62       62              
  Lines        7607     7606       -1     
  Branches     1215     1214       -1     
==========================================
+ Hits         5368     5373       +5     
+ Misses       1873     1865       -8     
- Partials      366      368       +2     

Impacted Files	Coverage Δ
jupyter_server/auth/login.py	68.93% <0.00%> (ø)
jupyter_server/auth/logout.py	44.44% <0.00%> (ø)
jupyter_server/auth/security.py	76.31% <0.00%> (ø)
jupyter_server/base/handlers.py	65.72% <0.00%> (ø)
jupyter_server/extension/manager.py	93.89% <ø> (ø)
jupyter_server/extension/serverextension.py	75.86% <0.00%> (ø)
jupyter_server/i18n/__init__.py	43.63% <0.00%> (ø)
jupyter_server/nbconvert/handlers.py	28.57% <0.00%> (ø)
jupyter_server/services/contents/filemanager.py	73.27% <ø> (ø)
jupyter_server/services/contents/manager.py	82.19% <ø> (ø)
... and 12 more

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