gh-94673: Isolate the _io module to Each Interpreter (gh-102663)

Aside from sys and builtins, _io is the only core builtin module that hasn't been ported to multi-phase init.  We may do so later (e.g. gh-101948), but in the meantime we must at least take care of the module's static types properly.  (This came up while working on gh-101660.)

https://github.com/python/cpython/issues/94673