Address flake8 E721 violations

[cottsay]

These violations don't show up in our CI right now because we've pinned flake8<6, but when that pin is dropped, we'll have enforcement for this rule.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c8995e3) 81.93% compared to head (34c2a1b) 81.93%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #580   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files          65       65           
  Lines        3753     3753           
  Branches      724      724           
=======================================
  Hits         3075     3075           
  Misses        622      622           
  Partials       56       56           

Files Changed	Coverage Δ
colcon_core/verb/__init__.py	100.00% <ø> (ø)
colcon_core/package_augmentation/__init__.py	100.00% <100.00%> (ø)
colcon_core/package_descriptor.py	100.00% <100.00%> (ø)

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