Revert "Adhere to PEP8"

This reverts commit aac0d52.

src/oemof/network/energy_system.py

@@ -171,7 +171,7 @@ def groups(self):
             (
                 g(n, gs)
                 for g in self._groupings
-                for n in self.nodes[self._first_ungrouped_node_index_:]
+                for n in self.nodes[self._first_ungrouped_node_index_ :]
             ),
             maxlen=0,
         )