[AIRFLOW-2778] Explicit import for dag_processing.list_py_file_paths

The use of utils.dag_processing.list_py_file_paths causes a failure if
utils.dag_processing is not already loaded indirectly.

airflow/models.py

@@ -79,6 +79,7 @@ from airflow.ti_deps.deps.trigger_rule_dep import TriggerRuleDep
 
 from airflow.ti_deps.dep_context import DepContext, QUEUE_DEPS, RUN_DEPS
 from airflow.utils import timezone
+from airflow.utils.dag_processing import list_py_file_paths
 from airflow.utils.dates import cron_presets, date_range as utils_date_range
 from airflow.utils.db import provide_session
 from airflow.utils.decorators import apply_defaults
@@ -520,7 +521,7 @@ class DagBag(BaseDagBag, LoggingMixin):
         stats = []
         FileLoadStat = namedtuple(
             'FileLoadStat', "file duration dag_num task_num dags")
-        for filepath in utils.dag_processing.list_py_file_paths(dag_folder):
+        for filepath in list_py_file_paths(dag_folder):
             try:
                 ts = timezone.utcnow()
                 found_dags = self.process_file(