Document `NewType.__supertype__`

[JelleZijlstra]

typing.NewType has always had an attribute __supertype__ pointing to its supertype. It is widely used in the wild (e.g. https://github.com/pydantic/pydantic/blob/00158878cb1c6218cd58ee62248db56c65e4172c/pydantic/_internal/_typing_extra.py#L153, https://github.com/quora/pyanalyze/blob/bd7f520adc2d8b098be657dfa514d1433bea3b0c/pyanalyze/annotations.py#L456), but not documented. Let's document it and commit to leaving it stable.

Linked PRs

• gh-104874: Document NewType.__supertype__ #104875
• [3.12] gh-104874: Document NewType.__supertype__ (GH-104875) #104906
• [3.11] gh-104874: Document NewType.__supertype__ (GH-104875) #104907