This improves the type-hints of numpy.dtypes for users and static-type checkers, in a backwards-compatible manner:

• Use Self for self types
• Ignored some (unavoidable) mypy errors (related to numpy.dtype being @final, and StringDType not having a scalar type)
• Mark stub-only private helper classes with @type_check_only
• Allow omitting the "itemsize" type-argument in (Bytes|Str|Void)DType by using type parameter defaults

See #27132 for details on the use of typing_extensions in .pyi stubs