Guzzle has deprecated the use of functions provided by their dependencies (as you can see here) in their latest versions.
I think it might make sense to upgrade the use of them in oauth1-client too, for example in EncodesUrl.php:

protected function createUrl($uri)
{
    return Psr7\uri_for($uri);
}

Becomes:

protected function createUrl($uri)
{
    return Psr7\Utils::uriFor($uri);
}

This will make the library future proof of any deprecation (and eventually give us the ability to use Mozart on it, as at the moment it will crash on those function calls).