Drupal 9 compatibility: RevisionLogEntityTrait

Problem/Motivation

I have enabled automated testing on Drupal 9 when commiting, and currently it is red: https://www.drupal.org/pift-ci-job/1809157

The error is the same for all tests:

Drupal\Tests\entity_share_async\Functional\EntityShareAsyncFunctionalTest
exception: [Other] Line 0 of sites/default/files/simpletest/phpunit-1.xml:
PHPunit Test failed to complete; Error: PHPUnit 8.5.5 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\entity_share_async\Functional\EntityShareAsyncFunctionalTest
E                                                                   1 / 1 (100%)

Time: 57.69 seconds, Memory: 4.00 MB

There was 1 error:

1) Drupal\Tests\entity_share_async\Functional\EntityShareAsyncFunctionalTest::testAsync
PHPUnit\Framework\Exception: Fatal error: Class Drupal\entity_share_client\Plugin\EntityShareClient\Processor\Revision contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Drupal\entity_share_client\Plugin\EntityShareClient\Processor\Revision::getEntityType) in /var/www/html/modules/contrib/entity_share/modules/entity_share_client/src/Plugin/EntityShareClient/Processor/Revision.php on line 28

/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:288
/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:171
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestSuite.php:597
/var/www/html/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:621
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:204
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:163

Caused by
ErrorException: unserialize(): Error at offset 0 of 418 bytes in /var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:272
Stack trace:
#0 [internal function]: PHPUnit\Util\PHP\AbstractPhpProcess::PHPUnit\Util\PHP\{closure}(8, 'unserialize(): ...', '/var/www/html/v...', 272, Array)
#1 /var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php(272): unserialize('\nFatal error: C...')
#2 /var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php(171): PHPUnit\Util\PHP\AbstractPhpProcess->processChildResult(Object(Drupal\Tests\entity_share_async\Functional\EntityShareAsyncFunctionalTest), Object(PHPUnit\Framework\TestResult), '\nFatal error: C...', '')
#3 /var/www/html/sites/simpletest/TestCase.php(761): PHPUnit\Util\PHP\AbstractPhpProcess->runTestJob('

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes