Load the pyproject.toml file.
Parameters:
use_pep517 - Has the user requested PEP 517 processing? None
means the user hasn't explicitly specified.
pyproject_toml - Location of the project's pyproject.toml file
setup_py - Location of the project's setup.py file
req_name - The name of the requirement we're processing (for
error reporting)
Returns:
None if we should use the legacy code path, otherwise a tuple
(
requirements from pyproject.toml,
name of PEP 517 backend,
requirements we should check are installed after setting
up the build environment
directory paths to import the backend from (backend-path),
relative to the project root.
)