Fix incorrect serialisation of the FixedTimezone (#38139)

This issue introduced in Airflow 2.8.

When the `start_date` of a DAG is specified with a fixed timezone, its serialisation will not be correct and this causes the scheduler to crash.

(cherry picked from commit 0720ea0)

airflow/serialization/serialized_objects.py

@@ -459,7 +459,7 @@ def serialize(
             return cls._encode(var.timestamp(), type_=DAT.DATETIME)
         elif isinstance(var, datetime.timedelta):
             return cls._encode(var.total_seconds(), type_=DAT.TIMEDELTA)
-        elif isinstance(var, Timezone):
+        elif isinstance(var, (Timezone, FixedTimezone)):
             return cls._encode(encode_timezone(var), type_=DAT.TIMEZONE)
         elif isinstance(var, relativedelta.relativedelta):
             return cls._encode(encode_relativedelta(var), type_=DAT.RELATIVEDELTA)

tests/serialization/test_dag_serialization.py

@@ -25,7 +25,7 @@
 import multiprocessing
 import os
 import pickle
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone as dt_timezone
 from glob import glob
 from pathlib import Path
 from typing import TYPE_CHECKING
@@ -686,7 +686,16 @@ def validate_deserialized_task(
                 datetime(2019, 7, 30, tzinfo=timezone.utc),
                 datetime(2019, 8, 1, tzinfo=timezone.utc),
             ),
-            (pendulum.datetime(2019, 8, 1, tz="UTC"), None, pendulum.datetime(2019, 8, 1, tz="UTC")),
+            (
+                datetime(2019, 8, 1, tzinfo=dt_timezone(timedelta(hours=1))),
+                datetime(2019, 7, 30, tzinfo=dt_timezone(timedelta(hours=1))),
+                datetime(2019, 8, 1, tzinfo=dt_timezone(timedelta(hours=1))),
+            ),
+            (
+                pendulum.datetime(2019, 8, 1, tz="UTC"),
+                None,
+                pendulum.datetime(2019, 8, 1, tz="UTC"),
+            ),
         ],
     )
     def test_deserialization_start_date(self, dag_start_date, task_start_date, expected_task_start_date):