PEP-518 wheel support

[gaborbernat]

We'll add PEP-518 support by exposing the following:

• option to replace python setup.py sdist with pip wheel to build packages,
• this builds wheel instead of sdist,
• we'll install wheels inside environments instead of sdists.

The benefit of doing this is that now users can specify their project dependencies for setuptools projects by using pyproject.toml.

Because sometimes sdist may be prefered and pip does not support building those yet; for now we'll expose it with an on-demand build = wheel tox config. By default build = sdist and this will keep doing python setup.py sdist. Once pip supports building sdist we'll replace python setup.py sdist with that command.

Will create the PR for this in the following days.

Full PEP-517 support (e.g. flint, etc is tracked by #573).

[rpkilby]

This may require changing some CLI and ini options. --sdistonly, skipsdist, and sdistsrc don't make as much sense given that we may be using wheels. Possibly add --distonly, skipdist, and distsrc, leaving the original for backwards compatibility?

[gaborbernat]

@rpkilby yeah I know, we'll need to shim them around. I would propose the word packaging instead of dist.