Drupal 9 support

drupal-check output:

 37/37 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

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  Line   tests/src/Unit/FormManagerTest.php                                                            
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  296    Call to deprecated method getMock() of class Drupal\Tests\UnitTestCase:                       
         in Drupal 8.5.0 and will be removed before Drupal 9.0.0.                                      
         Use \Drupal\Tests\PhpunitCompatibilityTrait::createMock() instead.                            
  296    Fetching class constant class of deprecated class Drupal\Core\Entity\EntityManagerInterface:  
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                                         
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  Line   tests/src/Unit/ResponseTest.php                                          
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  79     Call to deprecated method getMock() of class Drupal\Tests\UnitTestCase:  
         in Drupal 8.5.0 and will be removed before Drupal 9.0.0.                 
         Use \Drupal\Tests\PhpunitCompatibilityTrait::createMock() instead.       
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 [ERROR] Found 3 errors