TriFlag improvements related to defaults

[dskrypa]

• Improved TriFlag by allowing default to match consts
• Fixed TriFlag to reject late default_cb registration when required=True

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.89%. Comparing base (c59b57a) to head (3b29417).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #175   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files          54       54           
  Lines        7084     7085    +1     
  Branches     1138     1138           
=======================================
+ Hits         6864     6865    +1     
  Misses        152      152           
  Partials       68       68           

Flag	Coverage Δ
unittests	96.88% <100.00%> (+<0.01%)	⬆️

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