Remove private _PyLong_New() function

[skirpichev]

With PEP 757 - one can be replaced by PyLongWriter API.

Despite this is a private function, it's used by some projects (e.g. Sagemath). Maybe we should first deprecate this function.

Linked PRs

• gh-127936: convert marshal module to use import/export API for ints (PEP 757) #128530