Add SymbolTableFactory to symtable stub (#5998)

This is not documented API, but AFAIR the typeshed policy is now that stubs should preferably reflect reality rather than solely documented API.

See https://github.com/python/cpython/blob/main/Lib/symtable.py#L21

stdlib/symtable.pyi

@@ -47,3 +47,8 @@ class Symbol(object):
     def is_namespace(self) -> bool: ...
     def get_namespaces(self) -> Sequence[SymbolTable]: ...
     def get_namespace(self) -> SymbolTable: ...
+
+class SymbolTableFactory(object):
+    def __init__(self) -> None: ...
+    def new(self, table: Any, filename: str) -> SymbolTable: ...
+    def __call__(self, table: Any, filename: str) -> SymbolTable: ...