Change numerous default values to ... (#1722)

This is the convention, and some default values (e.g. strings) apparently
break pytype.

stdlib/3/urllib/request.pyi

@@ -68,7 +68,7 @@ class Request:
 
 class OpenerDirector:
     def add_handler(self, handler: BaseHandler) -> None: ...
-    def open(self, url: Union[str, Request], data: Optional[bytes] = None,
+    def open(self, url: Union[str, Request], data: Optional[bytes] = ...,
              timeout: float = ...) -> _UrlopenRet: ...
     def error(self, proto: str, *args: Any) -> _UrlopenRet: ...
 
@@ -159,7 +159,7 @@ class HTTPHandler(BaseHandler):
     def http_open(self, req: Request) -> _UrlopenRet: ...
 
 class HTTPSHandler(BaseHandler):
-    def __init__(self, debuglevel: int = 0,
+    def __init__(self, debuglevel: int = ...,
                  context: Optional[ssl.SSLContext] = ...,
                  check_hostname: bool = ...) -> None: ...
     def https_open(self, req: Request) -> _UrlopenRet: ...