Added

• Added ability to retry on connect or request timeout (retry_on_timeout option)
• Added better tests for retry callback

Changed

• Changed callback signature for on_retry_callback callback. Response object is no longer guaranteed to be present, so the callback signature now looks like this: (int $retryCount, int $delayTimeout, RequestInterface $request, array $options, ResponseInterface|null $response).
• Updated Guzzle requirement to v6.3 or newer

Fixed

• Clarified and cleaned up some documentation in README, including a typo.