If there are python library requirements set, append the python dir
        to the path.

def load_requirements(self):
        """If there are python library requirements set, append the python dir
        to the path."""

        for module_name, pip_name in iteritems(self.metadata.requirements):
            extant = self.dataset.config.requirements[module_name].url

            force = (extant and extant != pip_name)

            self._library.install_packages(module_name, pip_name, force=force)

            self.dataset.config.requirements[module_name].url = pip_name

        python_dir = self._library.filesystem.python()
        sys.path.append(python_dir)