Fix List item_type validation rejecting subclasses with a custom metaclass

[maximlt]

_validate_item_type used type(v) is not type to check that an item is a class. This strict identity check only holds for classes whose metaclass is
exactly type, so valid subclasses defined with a custom metaclass were wrongly rejected. Use isinstance(v, type) instead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.73%. Comparing base (bd43b28) to head (d6ad90c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1166   +/-   ##
=======================================
  Coverage   86.73%   86.73%           
=======================================
  Files           9        9           
  Lines        5321     5321           
=======================================
  Hits         4615     4615           
  Misses        706      706           

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