Specifically exclude flake8>=6

[cottsay]

colcon/ci#11 worked fine for the rest of the colcon repositories, but colcon-core also performs a bootstrap test which is having this same problem. Rather than employ another constraints.txt, it's easier and more correct to express the constraint in the dependency itself.

...until we come up with a long-term solution, that is.

[codecov]

Codecov Report

Base: 81.51% // Head: 81.51% // No change to project coverage 👍

Coverage data is based on head (c0e8c6d) compared to base (ac820e2).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #537   +/-   ##
=======================================
  Coverage   81.51%   81.51%           
=======================================
  Files          59       59           
  Lines        3381     3381           
  Branches      656      656           
=======================================
  Hits         2756     2756           
  Misses        579      579           
  Partials       46       46           

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