ast: fix stubtest issues (#7877)

Add annotations for `_ast.Tuple.dims` and `ast.main()`. Add allowlist comments for others.

stdlib/ast.pyi

@@ -259,3 +259,6 @@ if sys.version_info >= (3, 8):
     def get_source_segment(source: str, node: AST, *, padded: bool = ...) -> str | None: ...
 
 def walk(node: AST) -> Iterator[AST]: ...
+
+if sys.version_info >= (3, 9):
+    def main() -> None: ...