ui: replaced deprecated tostring() by tobytes()

> Deprecated since version 3.2, will be removed in version 3.9.

And they did!

https://docs.python.org/3.9/library/array.html#array.array.tobytes

ui/opensnitch/service.py

@@ -82,7 +82,7 @@ def _setup_interfaces(self):
             0x8912,  # SIOCGIFCONF
             struct.pack('iL', bytes, names.buffer_info()[0])
         ))[0]
-        namestr = names.tostring()
+        namestr = names.tobytes()
         self._interfaces = {}
         for i in range(0, outbytes, 40):
             name = namestr[i:i+16].split(b'\0', 1)[0]