Type-annotate SkipMixin and BaseXCom (#20011)

(cherry picked from commit 6dd0a0d)

airflow/models/skipmixin.py

@@ -17,7 +17,7 @@
 # under the License.
 
 import warnings
-from typing import TYPE_CHECKING, Iterable, Union
+from typing import TYPE_CHECKING, Iterable, Optional, Sequence, Union
 
 from airflow.models.taskinstance import TaskInstance
 from airflow.utils import timezone
@@ -26,6 +26,7 @@
 from airflow.utils.state import State
 
 if TYPE_CHECKING:
+    from pendulum import DateTime
     from sqlalchemy import Session
 
     from airflow.models import DagRun
@@ -66,9 +67,9 @@ def _set_state_to_skipped(self, dag_run: "DagRun", tasks: "Iterable[BaseOperator
     def skip(
         self,
         dag_run: "DagRun",
-        execution_date: "timezone.DateTime",
-        tasks: "Iterable[BaseOperator]",
-        session: "Session" = None,
+        execution_date: "DateTime",
+        tasks: Sequence["BaseOperator"],
+        session: "Session",
     ):
         """
         Sets tasks instances to skipped from the same dag run.
@@ -114,11 +115,7 @@ def skip(
         session.commit()
 
         # SkipMixin may not necessarily have a task_id attribute. Only store to XCom if one is available.
-        try:
-            task_id = self.task_id
-        except AttributeError:
-            task_id = None
-
+        task_id: Optional[str] = getattr(self, "task_id", None)
         if task_id is not None:
             from airflow.models.xcom import XCom