Allow any iterable to be passed to method_decorator

From reading the source of Django 2.1.9, this seems to be what's
allowed there.

Fixes https://github.com/mkurnikov/django-stubs/issues/78.

Author: PeterJCLaw

django-stubs/utils/decorators.pyi

@@ -1,11 +1,11 @@
-from typing import Any, Callable, Optional, Set, Tuple, Type, Union
+from typing import Any, Callable, Iterable, Optional, Type, Union
 
 from django.utils.deprecation import MiddlewareMixin
 
 class classonlymethod(classmethod): ...
 
 def method_decorator(
-    decorator: Union[Callable, Set[Callable], Tuple[Callable, Callable]], name: str = ...
+    decorator: Union[Callable, Iterable[Callable]], name: str = ...
 ) -> Callable: ...
 def decorator_from_middleware_with_args(middleware_class: Type[MiddlewareMixin]) -> Callable: ...
 def decorator_from_middleware(middleware_class: Type[MiddlewareMixin]) -> Callable: ...