Fix crash when using enumerate with start and a class attribute

doc/whatsnew/fragments/7821.bugfix

@@ -0,0 +1,3 @@
+Fixes bug that resulted in crash during ``unnecessary_list_index_lookup`` check.
+
+Closes #7821

pylint/checkers/refactoring/refactoring_checker.py

@@ -2306,15 +2306,13 @@ def _enumerate_with_start(
         return False, confidence
 
     def _get_start_value(self, node: nodes.NodeNG) -> tuple[int | None, Confidence]:
-        confidence = HIGH
-
-        if isinstance(node, (nodes.Name, nodes.Call)):
+        if isinstance(node, (nodes.Name, nodes.Call, nodes.Attribute)):
             inferred = utils.safe_infer(node)
             start_val = inferred.value if inferred else None
-            confidence = INFERENCE
-        elif isinstance(node, nodes.UnaryOp):
-            start_val = node.operand.value
-        else:
-            start_val = node.value
+            return start_val, INFERENCE
+        if isinstance(node, nodes.UnaryOp):
+            return node.operand.value, HIGH
+        if isinstance(node, nodes.Const):
+            return node.value, HIGH
 
-        return start_val, confidence
+        return None, HIGH

tests/functional/u/unnecessary/unnecessary_list_index_lookup.py

@@ -138,3 +138,14 @@ def return_start(start):
 
 for idx, val in enumerate():
     print(my_list[idx])
+
+class Command:
+    def _get_extra_attrs(self, extra_columns):
+        self.extra_rows_start = 8  # pylint: disable=attribute-defined-outside-init
+        for index, column in enumerate(extra_columns, start=self.extra_rows_start):
+            pass
+
+Y_START = 2
+nums = list(range(20))
+for y, x in enumerate(nums, start=Y_START + 1):
+    pass