Default to PEP 517-based builds (#843)
## Summary
Our current setup uses the legacy `setup.py`-based builds if a
`pyproject.toml` file isn't present. This matches pip's behavior.
However, `pypa/build` uses PEP 517-based builds in such cases, and it
looks like pip plans to make that the default
(https://github.com/pypa/pip/issues/9175), with the limiting factor
being performance issues related to isolated builds.
This is now the default behavior, but the `--legacy-setup-py` flag
allows users to opt-in to using `setup.py` directly for distributions
that lack a `pyproject.toml`.
