Fix ValueError: invalid literal for int() with base 16

[h]

The identifier() function previously returned the serial number separated with an underscore "_", whereas BrotherQLBackendPyUSB.init() expects the device_specifier to separate the serial number with a forward slash "/". This was causing the following error to be raised when a serial number is present for the printer:

ValueError: invalid literal for int() with base 16

This is fixed by PR #141 . @pklaus could you please review & merge?

[FriedrichFroebel]

Given the general maintenance state of this repository and the last account activity being in June 2021, I do not think that repeatedly pinging would have any real effect here.