Installation failed - Laravel 8

[mathmul]

Setup

$ php --version
PHP 7.4.10 (cli) (built: Sep  1 2020 16:52:39) ( ZTS Visual C++ 2017 x64 )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
$ php artisan --version
Laravel Framework 8.0.4

Issue

Running

composer require alexcode/laravel-sumup

resulted in

Installation failed, reverting ./composer.json to its original content.

More info

The last line before failure was

- Installation request for guzzlehttp/guzzle (locked at 7.0.1, required as ^7.0.1) -> satisfiable by guzzlehttp/guzzle[7.0.1].

Recreate issue

$ laravel new myapp
$ cd myapp
$ composer require alexcode/laravel-sumup

[mathmul]

Retried

composer require alexcode/laravel-sumup

now with

$ php artisan --version
Laravel Framework 7.27.0

and the installation was successful, but with a warning (yellow):

Package phpunit/php-token-stream is abandoned, you should avoid using it. No replacement was suggested.

The very same package is probably at fault for Laravel 8 IMHO.