Restore pyproject.toml handling to its version 19.0.3 state.
Restore pyproject.toml handling to its version 19.0.3 state.

rouge8/pip,pradyunsg/pip,pypa/pip,xavfernandez/pip,pfmoore/pip,xavfernandez/pip,sbidoul/pip,rouge8/pip,sbidoul/pip,pradyunsg/pip,pfmoore/pip,pypa/pip,rouge8/pip,xavfernandez/pip