fix some abstract classes in third_party/ (#1486)

stdlib/3/http/cookiejar.pyi

@@ -1,5 +1,3 @@
-# Stubs for http.cookiejar (Python 3.4)
-
 from typing import Iterable, Iterator, Optional, Sequence, Tuple, TypeVar, Union, overload
 from http.client import HTTPResponse
 import sys
@@ -28,6 +26,7 @@ class CookieJar(Iterable['Cookie']):
               name: str = ...) -> None: ...
     def clear_session_cookies(self) -> None: ...
     def __iter__(self) -> Iterator['Cookie']: ...
+    def __len__(self) -> int: ...
 
 class FileCookieJar(CookieJar):
     filename = ...  # type: str