Support Python 3.8 in hs_build_address.py

The Python code is such a nice addition to the documentation and the C
code for better understanding of onion v3 address generation. Straight
to the point and easy to understand.

Unfortunately it did not work with my distribution's Python version. I
have adjusted the code to support Python 3.8 (tested with 3.8.6) and
to still be compatible with Python 2.