Load migration file as module.

def load_migration_file(self, filename):
        """Load migration file as module."""
        path = os.path.join(self.directory, filename)
        # spec = spec_from_file_location("migration", path)
        # module = module_from_spec(spec)
        # spec.loader.exec_module(module)
        module = imp.load_source("migration", path)
        return module