Implement build system based on PEP 517 and 518

[albireox]

Python and pip are slowly rolling out the implementation for PEP 517 and 518. Here's an article describing it.

In particular this allows using tools such as poetry that provide a really nice workflow and remove many of the inconveniences of using requirements.txt, setup.py, etc.

The disadvantages are that the system is still being deployed (although pip already supports it) and that removes old-style installation from source with setup.py. The new system seems to be take-or-leave it and it's probably not worth it to try to implement it while keeping compatibility with the previous build system.

[albireox]

Poetry seems to have some issues related with pip not yet supporting PEP-517 (see python-poetry/poetry/issues/1279). At this point it would probably be best to implement #18, including changing setup.py.