message: Use a broader definition for the "modified" list when extracting block variables, by looking at Scope.bound instead of Scope.modified. Most of the time these are identical, but the latter includes symbols captured by e.g. `del`.
diff: @@ -212,7 +212,7 @@ class ControlFlowTransformer(converter.Base):
  orelse_scope = anno.getanno(node, annos.NodeAnno.ORELSE_SCOPE)
  cond_vars, undefined, nouts = self._get_block_vars(
  - node, body_scope.modified | orelse_scope.modified)
  + node, body_scope.bound | orelse_scope.bound)
  undefined_assigns = self._create_undefined_assigns(undefined)
  @@ -267,7 +267,7 @@ class ControlFlowTransformer(converter.Base):
  node = self.generic_visit(node)
  body_scope = anno.getanno(node, annos.NodeAnno.BODY_SCOPE)
  - loop_vars, undefined, _ = self._get_block_vars(node, body_scope.modified)
  + loop_vars, undefined, _ = self._get_block_vars(node, body_scope.bound)
  undefined_assigns = self._create_undefined_assigns(undefined)
  @@ -319,7 +319,7 @@ class ControlFlowTransformer(converter.Base):
  iter_scope = anno.getanno(node, annos.NodeAnno.ITERATE_SCOPE)
  loop_vars, undefined, _ = self._get_block_vars(
  - node, body_scope.modified | iter_scope.modified)
  + node, body_scope.bound | iter_scope.bound)
  undefined_assigns = self._create_undefined_assigns(undefined)
  