Correct superclass constructors in 3to4.

[rcorre]

Fixes #70542.

The 3to4 conversion tool was not handling superclass constructors.
We should translate the godot3 syntax:

func _init(a,b,c).(a,b,c):
    pass

func _init(a,b,c):
    super(a,b,c)

Originally, the _init conversion was intended to remove void return types from _init functions, as this was disallowed due to #50589.
As that was resolved by #53366, I removed that part of the conversion logic. If a void return type is present on a constructor, the converter now leaves it.

Here's a sample diff from my own project:

@@ -103,10 +105,11 @@ class Real:
 class Text:
        extends Setting

-       var choices: PoolStringArray
-       var value: String setget set_value, get_value
+       var choices: PackedStringArray
+       var value: String : get = get_value, set = set_value

-       func _init(section: String, key: String, default: String, choice_list: Array).(section, key, default) -> void:
+       func _init(section: String, key: String, default: String, choice_list: Array) -> void:
+               super(section, key, default)
                choices = choice_list

        func normalize(val):
@@ -129,9 +132,10 @@ class Text:
 class Boolean:
        extends Setting

-       var value: bool setget set_value, get_value
+       var value: bool : get = get_value, set = set_value

-       func _init(section: String, key: String, default: bool).(section, key, default) -> void:
+       func _init(section: String, key: String, default: bool) -> void:
+               super(section, key, default)
                pass

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.1.