Drupal 9 compatibility

Problem/Motivation

We found ourselves needing this module to be Drupal 9 compatible, Drupal 8's eol is near November 2nd 2021.
We run Update Status to see the minimum needed to support that:

FILE: modules/contrib/micro_node/tests/src/Functional/NodeAccessAndUrlTest.php
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 17   Drupal\Tests\BrowserTestBase::$defaultTheme is required. See
                    https://www.drupal.org/node/3083055, which includes
                    recommendations on which theme to use.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_node/tests/src/Functional/NodeAccessRecordsTest.php
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 17   Drupal\Tests\BrowserTestBase::$defaultTheme is required. See
                    https://www.drupal.org/node/3083055, which includes
                    recommendations on which theme to use.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_node/tests/src/Functional/NodeCreateTest.php
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 16   Drupal\Tests\BrowserTestBase::$defaultTheme is required. See
                    https://www.drupal.org/node/3083055, which includes
                    recommendations on which theme to use.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_node/tests/src/Functional/NodeEditRedirectTest.php
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 18   Drupal\Tests\BrowserTestBase::$defaultTheme is required. See
                    https://www.drupal.org/node/3083055, which includes
                    recommendations on which theme to use.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_node/micro_node.info.yml
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 0    Value of core_version_requirement: ^8.8 is not compatible
                    with the next major version of Drupal core. See
                    https://drupal.org/node/3070687.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_node/composer.json
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 0    The drupal/core requirement is not compatible with the next
                    major version of Drupal. Either remove it or update it to be
                    compatible. See
                    https://drupal.org/node/2514612#s-drupal-9-compatibility.
--------------------------------------------------------------------------------

Proposed resolution

Create a patch containing the above recommendations.

I'll try to do it soon!