Regression: `types.NotImplementedType` considered `@type_check_only`

[randolf-scholz]

#14717 marked _NotImplementedType as @type_check_only, this causes typing errors when types.NotImplementedType is used in runtime context Code sample in pyright playground

from types import NotImplementedType

assert isinstance(NotImplemented, NotImplementedType)
# "_NotImplementedType" is marked as @type_check_only and can be used only in type annotations  (reportGeneralTypeIssues)

(this checked fine with pyright 1.1.405 which uses older stub version)

[srittau]

It probably makes sense to move _NotImplementedType to types, renaming it, and removing the @type_check_only marker. I'm not sure why it is in builtins.pyi in the first place, possibly for historic reasons.