I think the easiest way is to save the value to another variable in _ready().
var full_health
func _ready():
full_health = health_points
This is a good method, because if you change health_points on a child class in the editor, it will save that value (meaning you can have different classes that all extend Actor and have their own max health).
However, you can also add an accessor function in the parent class, that returns the value you want.
For example, in the Actor class, add this function:
var health_points = 10
func get_health():
return health_points
In the Hero class:
print("full health: ", .get_health())
Notice the dot before the function call. This allows you to access the script you are extending.
Hope that helps.