Drupal 9 readiness

The code is in the dev branch

For those of us trying to upgrade now:

THIS IS THE WAY:
composer require 'drupal/contribute:1.x-dev@dev'

Or check for a newer dev release on the project page.

I thought this simple advice was worth spelling out, because some may habitually use the trick of using cweagans/composer-patches to apply patches via composer.json like so:

DON'T DO THIS! THIS IS NOT THE WAY:

    "extra": {
        "enable-patching": true,
        "patches": {
            "drupal/contribute": {
                "Drupal 9 compatibility patch": "https://www.drupal.org/files/issues/2021-05-03/entity_activity-drupal_9_readiness-3129711-7.patch"
            }
        },

There's a tricky reason why you can't use the patch here:
https://www.mediacurrent.com/blog/how-fix-catch-22-problem-drupal-9-fixe...

Short version: Composer can only patch a package that it can select for installation, so the core_version_requirement: ^8 || ^9 cannot be added by patching via composer.json.

In this case, just install the latest dev version:

composer require 'drupal/contribute:1.x-dev@dev'

... and you'll be ready for Drupal 9.

Please provide a Drupal 9 version on project page!
Until then this issue should remain open.

@joshua1234511 will be better to create a new issue and a new patch for your PR