For those of you who participated in the PEP 517 discussion during the summer of 2017 (just prior to its provisional acceptance), I want to flag that one of the issues discussed back then has now resurfaced for discussion. This is because the feature was turned on by default in pip's latest release (19.0) less than a week ago.
The issue is the one around whether the source directory should be included in sys.path. The resurfacing is more or less as predicted. For example, in one email  from August 29, 2017, Nick summarized the state of things by saying (not too long before provisional acceptance)--
So I think we can deem this one resolved in favour of "Frontends must
ensure the current directory is *NOT* on sys.path before importing the designated backend", as starting there will mean we maximise our chances of learning something new as part of the initial rollout of the provisionally accepted API.
2. If omitting it is genuinely a problem, we'll likely find out soon enough
as part of implementing a setup.py backend
Basically, that "soon enough" moment has arrived, and at least one discussion on how to resolve it has started on the tracker here: https://github.com/pypa/pip/issues/6163
There is another discussion here, but it's probably better for the discussion to be in one spot: https://github.com/pypa/setuptools/issues/1642 Maybe Discourse?