Add dedicated Arguments type

[jwodder]

This will make type-checking easier later.

This PR builds on #85.

[codecov]

Codecov Report

Attention: Patch coverage is 51.85185% with 13 lines in your changes missing coverage. Please review.

Project coverage is 86.79%. Comparing base (313675d) to head (d4b1953).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/duct.py	51.85%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   86.45%   86.79%   +0.33%     
==========================================
  Files           1        1              
  Lines         251      265      +14     
  Branches       39       41       +2     
==========================================
+ Hits          217      230      +13     
- Misses         26       27       +1     
  Partials        8        8              

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[asmacdo]

@jwodder might need to rebase this one, unless rebase and merge is smart enough to only include the last commit

[asmacdo]

well nevermind, rebase and merge is smart!