Add PyUnstable compat API for PyUnstable_Object_IsUniquelyReferenced and PyUnstable_SetImmortal

[luxedo]

In #5828 we added bindings for

• PyUnstable_Object_IsUniquelyReferenced Python 3.14
• PyUnstable_SetImmortal Python 3.15

We could add compatibility for them given these reference implementations:

• Add PyUnstable_Object_IsUniquelyReferenced python/pythoncapi-compat#149
• Add PyUnstable_SetImmortal python/pythoncapi-compat#164