Update pi-thermal-printer docs

[johnchowork]

I'm assuming that some folks are coming to this package from Adafruit's pi-thermal-printer documentation.
This package has been updated to use python3

This means that the command to run the test should be python3 printertest.py as written in the Readme here.
But this also means that the package to be installed should be changed to python3-serial meaning that you need to run apt-get install python3-serial to install that package.

[gregorspamsa]

Thanks so much! I kept on getting the error below and installing the python3-serial package fixed the issue 🙇

Traceback (most recent call last):
  File "printertest.py", line 3, in <module>
    from Adafruit_Thermal import *
  File "/home/pi/Python-Thermal-Printer_orig/Adafruit_Thermal.py", line 35, in <module>
    from serial import Serial
ModuleNotFoundError: No module named 'serial'