[3.13] gh-113993: For string interning, do not rely on (or assert) _Py_IsImmortal (GH-121358) (GH-121851)

gh-113993: For string interning, do not rely on (or assert) _Py_IsImmortal (GH-121358)

Older stable ABI extensions are allowed to make immortal objects mortal.
Instead, use `_PyUnicode_STATE` (`interned` and `statically_allocated`).
(cherry picked from commit 956270d08d5c23f59937e2f29f8e0b7f63d68afd)

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