1.3.1 release is missing a `[build-system]` section

[mgorny]

The 1.3.1 release seems to have switched from setup.py to pyproject.toml project config. However, it lacks a build-system section as required by PEP 517 and as such is not valid (and works for some people only because of lenient implementation in pip).

[C0rn3j]

works for some people

Under which circumstance does it not work, or does it upset some linter or some such?

Not trying to be snarky, just curious as I have not run into any issues building such projects with either pip nor Python's build module.