Prior to this change, generation of the character lookup table could use
Python 2 or Python 3, depending on what the first instance of python in
the PATH pointed to. On most systems, python is a symbolic link to
python2.7. However, given that Python 2 is EOL, it makes sense to switch
the code to use Python 3 now, rather than later.
This change updates the requirements to use Python 3.6 or later, and
updates the Makefile to invoke the script with the detected python
binary, rather than relying on the shebang to use the system Python.
nirenjan