Load the pyproject.toml file.

        After calling this routine, all of the attributes related to PEP 517
        processing for this requirement have been set. In particular, the
        use_pep517 attribute can be used to determine whether we should
        follow the PEP 517 or legacy (setup.py) code path.

def load_pyproject_toml(self):
        # type: () -> None
        """Load the pyproject.toml file.

        After calling this routine, all of the attributes related to PEP 517
        processing for this requirement have been set. In particular, the
        use_pep517 attribute can be used to determine whether we should
        follow the PEP 517 or legacy (setup.py) code path.
        """
        pep517_data = load_pyproject_toml(
            self.use_pep517,
            self.pyproject_toml,
            self.setup_py,
            str(self)
        )

        if pep517_data is None:
            self.use_pep517 = False
        else:
            self.use_pep517 = True
            requires, backend, check = pep517_data
            self.requirements_to_check = check
            self.pyproject_requires = requires
            self.pep517_backend = Pep517HookCaller(self.setup_py_dir, backend)

            # Use a custom function to call subprocesses
            self.spin_message = ""

            def runner(cmd, cwd=None, extra_environ=None):
                with open_spinner(self.spin_message) as spinner:
                    call_subprocess(
                        cmd,
                        cwd=cwd,
                        extra_environ=extra_environ,
                        show_stdout=False,
                        spinner=spinner
                    )
                self.spin_message = ""

            self.pep517_backend._subprocess_runner = runner