Export PixelIDValueEnum members as members of Enum in Python binding

[Czaki]

Is the feature request related to a problem? Please describe.

Currently, members of PixelIDValueEnum Enum are exported on the module level. Currently modern python packages export enum values as members of the class, not on the module level. So newcomers may be confused. It will be nice to also export PixelIDValueEnum members in SimpleITK.PixelIDValueEnum.member

Describe the proposed solution

Add something like

class PixelIDValueEnum(IntEnum):
    sitkUInt8 = 1
    ...

Or even provide also numpy like aliases:

class PixelIDValueEnum(IntEnum):
    sitkUInt8 = 1
    uint8 = 1
    sitkUInt16 = 3
    uint16 = 3
    ...

Additional context

#1991 (comment)

Maybe this change may be also nice in other languages, but I'm not enough familiar with their conventions.