first version with linter fixes

src/Service/ExternalApi.php

-<?php
-
-declare(strict_types=1);
-
-namespace Dbp\Relay\CourseBundle\Service;
-
-use Dbp\Relay\CourseBundle\Entity\Course;
-
-class ExternalApi implements CourseProviderInterface
-{
-    private $courses;
-
-    public function __construct(MyCustomService $service)
-    {
-        // Make phpstan happy
-        $service = $service;
-
-        $this->courses = [];
-        $course1 = new Course();
-        $course1->setIdentifier('graz');
-        $course1->setName('Graz');
-
-        $course2 = new Course();
-        $course2->setIdentifier('vienna');
-        $course2->setName('Vienna');
-
-        $this->courses[] = $course1;
-        $this->courses[] = $course2;
-    }
-
-    public function getCourseById(string $identifier): ?Course
-    {
-        foreach ($this->courses as $course) {
-            if ($course->getIdentifier() === $identifier) {
-                return $course;
-            }
-        }
-
-        return null;
-    }
-
-    public function getCourses(): array
-    {
-        return $this->courses;
-    }
-}

src/Service/MyCustomService.php

-<?php
-
-declare(strict_types=1);
-
-namespace Dbp\Relay\CourseBundle\Service;
-
-class MyCustomService
-{
-    private $someConfig;
-
-    public function __construct(string $someConfig)
-    {
-        $this->$someConfig = $someConfig;
-    }
-}

tests/Service/ExternalApiTest.php

@@ -4,23 +4,15 @@ declare(strict_types=1);
 
 namespace Dbp\Relay\CourseBundle\Tests\Service;
 
-use Dbp\Relay\CourseBundle\Service\ExternalApi;
-use Dbp\Relay\CourseBundle\Service\MyCustomService;
 use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
 
 class ExternalApiTest extends WebTestCase
 {
-    private $api;
-
     protected function setUp(): void
     {
-        $service = new MyCustomService('test-42');
-        $this->api = new ExternalApi($service);
     }
 
     public function test()
     {
-        $this->assertTrue(true);
-        $this->assertNotNull($this->api);
     }
 }