Add type for undocumented copyreg.dispatch_table (#5959)

stdlib/copyreg.pyi

@@ -14,3 +14,5 @@ def constructor(object: Callable[[_Reduce[_TypeT]], _TypeT]) -> None: ...
 def add_extension(module: Hashable, name: Hashable, code: SupportsInt) -> None: ...
 def remove_extension(module: Hashable, name: Hashable, code: int) -> None: ...
 def clear_extension_cache() -> None: ...
+
+dispatch_table: dict[type, Callable[[type], str | _Reduce[type]]]  # undocumented