Fix type error in retryHandler with ConnectException

Since guzzle 7.0 the ConnectException no longer extends RequestException. 
This leads to type errors in the retry handler in case of connection exceptions.
Change the signature to GuzzleException to cover all possible cases

src/Client.php

@@ -267,7 +267,7 @@ public function unwrap(array $promises): array
     private function getRetryHandler(): Closure
     {
         return function ($retries, ?RequestInterface $request, ?ResponseInterface $response,
-            ?RequestException $exception) {
+            ?GuzzleException $exception) {
             if (!$response || $response->getStatusCode() > 500 || ($exception && $exception instanceof ConnectException))
                 return false;