Make final classes that can be

This allows us to adapt to upstream changes without that being a
breaking change.

UPGRADE.md

@@ -1,6 +1,11 @@
 UPGRADE to 4.0
 ==============
 
+## Final classes
+
+Some classes have been marked as `final` because they are not supposed to be
+extended. Consider using composition instead of inheritance.
+
 ## Mandatory ManagerRegistry argument
 
 It is now mandatory to pass a `ManagerRegistry` instance to the constructor of

src/Command/LoadDataFixturesDoctrineCommand.php

@@ -25,7 +25,7 @@
 /**
  * Load data fixtures from bundles.
  */
-class LoadDataFixturesDoctrineCommand extends DoctrineCommand
+final class LoadDataFixturesDoctrineCommand extends DoctrineCommand
 {
     /** @param PurgerFactory[] $purgerFactories */
     public function __construct(

src/DependencyInjection/DoctrineFixturesExtension.php

@@ -13,7 +13,7 @@
 
 use function dirname;
 
-class DoctrineFixturesExtension extends Extension
+final class DoctrineFixturesExtension extends Extension
 {
     /**
      * {@inheritDoc}

src/DoctrineFixturesBundle.php

@@ -11,7 +11,7 @@
 
 use function dirname;
 
-class DoctrineFixturesBundle extends Bundle
+final class DoctrineFixturesBundle extends Bundle
 {
     public function build(ContainerBuilder $container): void
     {