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

Python 2.7 has already retired and it will no longer be maintained therefore it will better to drop the support of Python 2. It will also include removal of test suite for scancode-toolkit and this project is an extension of my previous year GSoC 2019 porting project. Also to add the support for python 3.7 and python 3.8 and to improve the third party package provision. Currently, we maintain a third party repository which locks all dependencies of scancode.But we face the following problems. a)We supported Python 2 & Python 3.6. Each version of python may need a pre-built wheel if there is native code. And to add support for python 3.7, 3.8 we have to add wheels in the third party repo of 4 variants(oses) for each dependency.
b)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 very long and tarballs too big. Therefore i decided to improve the provision of third party dependencies to minimise the problem we facing till now.