Add docutils.nodes.system_message and .BackLinkable

Add stubs for docutils.nodes.system_message and docutils.nodes.BackLinkable

Author: adamtheturtle

stubs/docutils/docutils/nodes.pyi

@@ -118,6 +118,9 @@ class TextElement(Element):
 class FixedTextElement(TextElement):
     def __init__(self, rawsource: str = "", text: str = "", *children: Node, **attributes) -> None: ...
 
+class BackLinkable:
+    def add_backref(self, refid: str) -> None: ...
+
 class Text(Node, str):
     tagname: ClassVar[str]
     children: tuple[()]
@@ -156,6 +159,10 @@ class literal_block(General, FixedTextElement): ...
 class substitution_definition(Special, Invisible, TextElement): ...
 class raw(Special, Inline, PreBibliographic, FixedTextElement): ...
 
+class system_message(Special, BackLinkable, PreBibliographic, Element):
+    def __init__(self, message: str | None = None, *children: Node, **attributes) -> None: ...
+    def astext(self) -> str: ...
+
 class NodeVisitor:
     def __init__(self, document: document): ...
     def __getattr__(self, __name: str) -> Incomplete: ...