[python] Builtin.open returns FileIO (with write methods and stuff)

std/python/lib/Builtin.hx

@@ -2,7 +2,7 @@
 package python.lib;
 
 
-import python.lib.io.IOBase;
+import python.lib.io.FileIO;
 import python.lib.Dict;
 import python.NativeIterable;
 import python.NativeIterator;
@@ -41,7 +41,7 @@ extern class Builtin {
 	@:overload(function (f:Tuple<Dynamic>):Int {})
 	public static function len(x:String):Int;
 
-	public static function open(file:String, mode:String, ?buffering:Int = -1, ?encoding:String = null, ?errors : String, ?newline:String, ?closefd:Bool, ?opener:String->Int->FileDescriptor):IOBase;
+	public static function open(file:String, mode:String, ?buffering:Int = -1, ?encoding:String = null, ?errors : String, ?newline:String, ?closefd:Bool, ?opener:String->Int->FileDescriptor):FileIO;
 
 	//public static function divmod():Void;
 	//public static function input():Void;