Upgrade stripe library to stripe/stripe-php version 6.*

You need to follow the instructions to install the Stripe PHP library - using composer whatever way works on your site.

Once you have composer installed, you have to follow the instructions here: https://www.drupal.org/node/2822349 to merge the module's composer.json file into the site's main composer.json file in the drupal root folder.

It's not particularly straight forward. Once you have achieved this though you should be able to install the module.

Having changed composer.json, you must make sure that you don't overwrite it, when you upgrade your drupal version, or you will break it again, and then probably spend another hour or so trying to remember how to fix it .. like I just did.

Yeah, I think this is valid. The version of Stripe required, as noted in the composer.json file is:

{
    "require": {
        "stripe/stripe-php": "3.*"
    }
}

That version is, at it's latest 3.x release 3 years old. The fact that the API is so old is a huge detractor for me in using this module -- and thus, by extension, ubercart.

The README of this project encourages safe practices but uses a woefully outdated Stripe API.

Good point. 8.x needs a version increase.

I'm currently focusing on 7.x for now, it was in the same outdated condition, i'm planning to work on 8.x next.

The stripe-php library Version has been increased to ^6.43.1 in 8.x-3.x..

There were no breaking changes based on the Stripe API calls used by the module.