bug #18589 [Serializer] fix parent class of AbstractNormalizer (xabbuh)

This PR was merged into the 3.1-dev branch.

Discussion
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[Serializer] fix parent class of AbstractNormalizer

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony/symfony#17545 (comment)
| License       | MIT
| Doc PR        |

We must maintain the parent class to not break BC.

Commits
-------

45b9ed9 fix parent class of AbstractNormalizer

Author: fabpot

Normalizer/AbstractNormalizer.php

@@ -25,10 +25,8 @@
  *
  * @author Kévin Dunglas <dunglas@gmail.com>
  */
-abstract class AbstractNormalizer implements NormalizerInterface, DenormalizerInterface, SerializerAwareInterface
+abstract class AbstractNormalizer extends SerializerAwareNormalizer implements NormalizerInterface, DenormalizerInterface, SerializerAwareInterface
 {
-    use SerializerAwareTrait;
-
     const CIRCULAR_REFERENCE_LIMIT = 'circular_reference_limit';
     const OBJECT_TO_POPULATE = 'object_to_populate';
     const GROUPS = 'groups';