[python] Initialize bigEndian field for NativeInput and NativeOutput (see #4444)

std/python/io/NativeInput.hx

@@ -34,6 +34,7 @@ class NativeInput<T:IOBase> extends Input{
 
 	function new (s:T) {
 		this.stream = s;
+		this.bigEndian = false;
 		wasEof = false;
 		if (!stream.readable()) throw "Write-only stream";
 	}

std/python/io/NativeOutput.hx

@@ -33,6 +33,7 @@ class NativeOutput<T:IOBase> extends Output {
 	public var canSeek(get_canSeek, null):Bool;
 
 	public function new (stream:T) {
+		this.bigEndian = false;
 		this.stream = stream;
 		if (!stream.writable()) throw "Read only stream";
 	}