Fix for string operation

* With Python version 3 data and text are treated different. Convert the
* text accordingly.

escpos/printer.py

@@ -159,6 +159,9 @@ class File(Escpos):
 
     def _raw(self, msg):
         """ Print any command sent in raw format """
+        if type(msg) is str:
+            self.device.write(msg.encode());
+        else:
             self.device.write(msg);