Installing a v8 module
You'll need to fork the module. This is more of a pain than patching but it's much better now that you can make issue forks on drupal.org.
Here's a detailed tutorial.
The basic workflow is this:

Make an issue fork on drupal.org that updates the version requirement.
Add the forked repo to your composer.json and update your site that way.
When the module gets a real Drupal 9 release, switch your composer.json back to the release.