fix: mypy linter fixes

src/library_analyzer/processing/api/purity_analysis/model/_purity.py

@@ -21,7 +21,7 @@ class PurityResult(ABC):
     def update(self, other: PurityResult) -> PurityResult:
         return self._update(other)
 
-    def _update(self, other: PurityResult) -> PurityResult:
+    def _update(self, other: PurityResult) -> PurityResult | None:
         """Update the current result with another result.
 
         Parameters

src/library_analyzer/processing/api/purity_analysis/model/_reference.py

@@ -75,11 +75,11 @@ def is_leaf(self) -> bool:
 class CallGraphForest:
     graphs: dict[str, CallGraphNode] = field(default_factory=dict)
 
-    def add_graph(self, graph_name, graph) -> None:
+    def add_graph(self, graph_name: str, graph: CallGraphNode) -> None:
         self.graphs[graph_name] = graph
 
-    def get_graph(self, graph_name) -> CallGraphNode:
+    def get_graph(self, graph_name: str) -> CallGraphNode | None:
         return self.graphs.get(graph_name)
 
-    def delete_graph(self, graph_name) -> None:
+    def delete_graph(self, graph_name: str) -> None:
         del self.graphs[graph_name]

src/library_analyzer/processing/api/purity_analysis/model/_scope.py

@@ -269,9 +269,6 @@ class FunctionScope(Scope):
     values: list[Scope | ClassScope] = field(default_factory=list)
     calls: list[Scope | ClassScope] = field(default_factory=list)
 
-    def __getitem__(self, item) -> Scope | ClassScope:
-        return self.values[item]
-
     def remove_call_node_by_name(self, name: str) -> None:
         for call in self.calls:
             if call.symbol.name == name: