Move tornado to 2and3 (#3780)

This also uses a trick to avoid `Incompatible with supertype` errors for `get()` etc. that used to have signature like `def get(*args, **kwars): ...` (that btw is used in tornado itself, see https://github.com/tornadoweb/tornado/blob/master/tornado/web.py#L266).

third_party/2and3/tornado/web.pyi

@@ -1,4 +1,6 @@
-from typing import Any
+import sys
+
+from typing import Any, Callable, Optional
 from tornado import httputil
 
 MIN_SUPPORTED_SIGNED_VALUE_VERSION: Any
@@ -6,6 +8,12 @@ MAX_SUPPORTED_SIGNED_VALUE_VERSION: Any
 DEFAULT_SIGNED_VALUE_VERSION: Any
 DEFAULT_SIGNED_VALUE_MIN_VERSION: Any
 
+if sys.version_info[:2] >= (3, 5):
+    from typing import Awaitable
+    _MethodType = Callable[..., Optional[Awaitable[None]]]
+else:
+    _MethodType = Callable[..., Any]
+
 class RequestHandler:
     SUPPORTED_METHODS: Any
     application: Any
@@ -14,16 +22,16 @@ class RequestHandler:
     path_kwargs: Any
     ui: Any
     def __init__(self, application, request, **kwargs) -> None: ...
-    def initialize(self): ...
+    initialize: Callable[..., None] = ...
     @property
     def settings(self): ...
-    def head(self, *args, **kwargs): ...
-    def get(self, *args, **kwargs): ...
-    def post(self, *args, **kwargs): ...
-    def delete(self, *args, **kwargs): ...
-    def patch(self, *args, **kwargs): ...
-    def put(self, *args, **kwargs): ...
-    def options(self, *args, **kwargs): ...
+    head: _MethodType
+    get: _MethodType
+    post: _MethodType
+    delete: _MethodType
+    patch: _MethodType
+    put: _MethodType
+    options: _MethodType
     def prepare(self): ...
     def on_finish(self): ...
     def on_connection_close(self): ...