Use pyproject.toml files for easier management

[rodolpheh]

This merge request is to introduce pyproject.toml files for a more advanced, yet easier management of the project.

List of changes:

• Fix MANIFEST.in files (all .py files in src are automatically included)
• Move cffi_build.py out of the src folder but add it in MANIFEST.in so it is included in sdist
• Remove cffi_src.h and change it for a simple line
• Richer description of the packages

This is also in the hopes that with pyproject.toml files comes the possibility to provide more build options (like using Poetry, or cibuildwheel)