Build/check should fallback to setuptools unconditionally

[jaraco]

In this comment, @gaborbernat suggests that the project should unconditionally fallback to setuptools if PEP 517 is not indicated.

That seems like a plausible suggestion, but I'd like to separate that concern from the concern in the PR.

[takluyver]

I've closed most of the issues relating to the build and check layer, but I'm still open to the simple change to make them fallback to setuptools, if people want to implement that. There's already a compat_system function implementing such a fallback.