Support Python 3.x Part 2

Goals

Remove legacy Python code
Support latest Python 3.x
Improve 3rd party package provisioning

ScanCode has been ported on Python 3 but still supports Python 2.7. The goal of this project is to remove all the code that provides Python 2-specific support.

In addition, the way we deal with 3rd party packages needs to be refactored: today we embed a copy of all our third-party dependencies as pre-built wheels in the ScanCode repository. This creates some problems:

We supported Python 2 and now support Python 3.6. Each version may need a pre-built wheel if there is native code. With 3.7, 3.8 and beyond this means creating and storing in the ScanCode git repo 4 variants for each dependency.
We support Windows, Linux, macOS and possibly FreeBSD each with some possible architecture variants. For each of these we are storing a variant for each of the Python versions above.

This creates way too many wheels that end up making ScanCode checkouts and tarballs too big. We need to define a new improved way to handle third-party dependencies including:

Make the build automation more prominent
Ensure secured pinning of wheel versions and actual files (e.g. using checksums)
Do not bundle these wheels anymore.

For more info see Disk quota and Limitations.

Basically it is an extension of previous GSoC 2019 Project: Port scancode-toolkit to Python 3