fix type signature for _RuleList

`List` is invariant, `Sequence` is covariant

tornado/routing.py

@@ -184,7 +184,7 @@ def request_callable(request):
 from tornado.log import app_log
 from tornado.util import basestring_type, import_object, re_unescape, unicode_type
 
-from typing import Any, Union, Optional, Awaitable, List, Dict, Pattern, Tuple, overload
+from typing import Any, Union, Optional, Awaitable, List, Dict, Pattern, Tuple, overload, Sequence
 
 
 class Router(httputil.HTTPServerConnectionDelegate):
@@ -286,7 +286,7 @@ def finish(self) -> None:
 
 # _RuleList can either contain pre-constructed Rules or a sequence of
 # arguments to be passed to the Rule constructor.
-_RuleList = List[
+_RuleList = Sequence[
     Union[
         "Rule",
         List[Any],  # Can't do detailed typechecking of lists.