gh-128844: Make `_Py_TryIncref` public as an unstable API. (#128926)

This exposes `_Py_TryIncref` as `PyUnstable_TryIncref()` and the helper
function `_PyObject_SetMaybeWeakref` as `PyUnstable_EnableTryIncRef`.

These are helpers for dealing with unowned references in a safe way,
particularly in the free threading build.

Co-authored-by: Petr Viktorin <encukou@gmail.com>