Add ClassVar annotations on Migration class attributes (#2530)

See also:

- astral-sh/ruff#5243
- #2524

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

Author: zyv

django-stubs/db/migrations/migration.pyi

@@ -1,17 +1,18 @@
 from collections.abc import Sequence
+from typing import ClassVar
 
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.migrations.operations.base import Operation
 from django.db.migrations.state import ProjectState
 from typing_extensions import Self
 
 class Migration:
-    operations: Sequence[Operation]
-    dependencies: Sequence[tuple[str, str]]
-    run_before: Sequence[tuple[str, str]]
-    replaces: Sequence[tuple[str, str]]
-    initial: bool | None
-    atomic: bool
+    operations: ClassVar[Sequence[Operation]]
+    dependencies: ClassVar[Sequence[tuple[str, str]]]
+    run_before: ClassVar[Sequence[tuple[str, str]]]
+    replaces: ClassVar[Sequence[tuple[str, str]]]
+    initial: ClassVar[bool | None]
+    atomic: ClassVar[bool]
     name: str
     app_label: str
     def __init__(self, name: str, app_label: str) -> None: ...