Add __slots__ to SubgraphView class

Author: InnovativeInventor

gerrychain/partition/assignment.py

@@ -17,10 +17,10 @@ class Assignment(Mapping):
     An :class:`Assignment` has a ``parts`` property that is a dictionary of the form
     ``{part: <frozenset of nodes in part>}``.
     """
-    __slots__ = (
+    __slots__ = [
         'parts',
-        'mapping',
-    )
+        'mapping'
+    ]
 
     def __init__(self, parts, mapping=None, validate=True):
         if validate:

gerrychain/partition/subgraphs.py

@@ -1,4 +1,10 @@
 class SubgraphView:
+    __slots__ = [
+        "graph",
+        "parts",
+        "subgraphs_cache"
+    ]
+
     def __init__(self, graph, parts):
         self.graph = graph
         self.parts = parts