Fix typing error in NetworkedPrinterOutputDevice.post

Strictly speaking the QNetworkManager.post() method takes a QByteArray instead of Python bytes, but according to the PyQt documentation, PyQt handles that conversion transparently:
http://pyqt.sourceforge.net/Docs/PyQt5/gotchas.html#python-strings-qt-strings-and-unicode

cura/PrinterOutput/NetworkedPrinterOutputDevice.py

@@ -213,7 +213,7 @@ class NetworkedPrinterOutputDevice(PrinterOutputDevice):
         request = self._createEmptyRequest(target)
         self._last_request_time = time()
         if self._manager is not None:
-            reply = self._manager.post(request, data)
+            reply = self._manager.post(request, data.encode())
             if on_progress is not None:
                 reply.uploadProgress.connect(on_progress)
             self._registerOnFinishedCallback(reply, on_finished)