[docutils] Fix type of standalone.Reader.document (#14447)

Author: adamtheturtle

stubs/docutils/docutils/readers/standalone.pyi

@@ -1,11 +1,11 @@
 from typing import ClassVar, Final, TypeVar
 
-from docutils import readers
+from docutils import nodes, readers
 
 __docformat__: Final = "reStructuredText"
 
 _S = TypeVar("_S", bound=str | bytes)
 
 class Reader(readers.Reader[_S]):
-    document: None  # type: ignore[assignment]
+    document: nodes.document | None  # type: ignore[assignment]
     config_section_dependencies: ClassVar[tuple[str, ...]]