linkify_issues: Fix mypy issue

src/linkify_issues.py:55: error: Argument 1 to "append" of "list" has incompatible type "reference"; expected "Text"  [arg-type]

src/linkify_issues.py

@@ -39,7 +39,7 @@ def condition(node: nodes.Node) -> bool:
 
         for node in findall(self.document)(condition):
             text = node.astext()
-            retnodes = []
+            retnodes: list[t.Union[nodes.reference, nodes.Text]] = []
             pos = 0
             for match in issue_re.finditer(text):
                 if match.start() > pos: