d96c18b3a8
## Summary This PR modifies `puffin-build` to be closer in behavior to [pip](https://github.com/pypa/pip/blob/a15dd75d98884c94a77d349b800c7c755d8c34e4/src/pip/_internal/pyproject.py#L53) and [build](https://github.com/pypa/build/blob/de5b44b0c28c598524832dff685a98d5a5148c44/src/build/__init__.py#L94). Specifically, if a project contains a `[build-system]` field, but no `build-backend`, we now perform a PEP 517 build (instead of using `setup.py` directly) _and_ respect the `requires` of the `[build-system]`. Without this change, we were failing to build source distributions for packages like `ujson`. Closes #527. --------- Co-authored-by: konstin <konstin@mailbox.org>