fix: add import statement for handlers

lms/djangoapps/certificates/apps.py

@@ -23,6 +23,7 @@ class CertificatesConfig(AppConfig):
         # Can't import models at module level in AppConfigs, and models get
         # included from the signal handlers
         from lms.djangoapps.certificates import signals  # pylint: disable=unused-import
+        from lms.djangoapps.certificates import handlers  # pylint: disable=unused-import
         if settings.FEATURES.get('ENABLE_SPECIAL_EXAMS'):
             from lms.djangoapps.certificates.services import CertificateService
             set_runtime_service('certificates', CertificateService())