nested_except_trace: fix false positives in "try", "else", "finally"

The except_clause which are in the "try", "else", or "finally" sections
were detected by mistake.

my2to3/fixes/fix_nested_except_trace.py

+from lib2to3.pygram import python_symbols as syms
+
 from my2to3.fixes import BaseFix
 from my2to3.trace import create_table
 
@@ -28,35 +30,24 @@ class FixNestedExceptTrace(BaseFix):
                       ['else' ':' (simple_stmt | suite)]
                       ['finally' ':' (simple_stmt | suite)]) >
   """
-  order = "pre"
-
-  def start_tree(self, tree, filename):
-    super(FixNestedExceptTrace, self).start_tree(tree, filename)
-
-    # Store the results of every node that matches the pattern
-    self.results = []
 
-  def find_parents_results(self, node):
-    parents_results = []
-    parent = node.parent
-    while parent is not None:
-      for r in self.results:
-        if r['node'] is parent:
-          parents_results.append(r)
-      parent = parent.parent
-    return parents_results
+  def list_except_clause_children(self, node):
+    for child in node.children:
+      if child.type == syms.except_clause:
+        yield child
+      # BBB: yield from
+      for c in self.list_except_clause_children(child):
+        yield c
 
   def transform(self, node, results):
-    self.results.append(results)
-
-    # Find if the current node has parents, which are also a try_stmt
-    for parent_results in self.find_parents_results(node):
-      # Check if the node and its parent use the same variable name for the
-      # caught exception
-      parent_exception_node = parent_results['cleanup'][0]
-      exception_node = results['cleanup'][0]
+    try:
+      except_clause, _, body = results['cleanup']
+    except ValueError:
+      return
+
+    for child in self.list_except_clause_children(body):
       try:
-        if exception_node.children[3] == parent_exception_node.children[3]:
-          trace(self.filename, parent_exception_node.get_lineno(), exception_node.get_lineno())
+        if except_clause.children[3] == child.children[3]:
+          trace(self.filename, except_clause.get_lineno(), child.get_lineno())
       except Exception:
         pass