Fix return type of werkzeug's ProxyFix (#3320)

Author: lalinsky

third_party/2and3/werkzeug/middleware/proxy_fix.pyi

@@ -20,4 +20,4 @@ class ProxyFix(object):
         x_prefix: int = ...,
     ) -> None: ...
     def get_remote_addr(self, forwarded_for: Iterable[str]) -> Optional[str]: ...
-    def __call__(self, environ: WSGIEnvironment, start_response: StartResponse) -> WSGIApplication: ...
+    def __call__(self, environ: WSGIEnvironment, start_response: StartResponse) -> Iterable[bytes]: ...