Fix imports.

django-stubs/db/models/expressions.pyi

@@ -1,23 +1,31 @@
 import datetime
 from decimal import Decimal
-from typing import Any, Callable, Dict, Iterable, Iterator, List, Optional, Sequence, Set, Tuple, Type, TypeVar, Union
+import sys
+from typing import (
+    Any, Callable, Dict, Iterable, Iterator, List, Optional, Sequence,
+    Set, Tuple, Type, TypeVar, Union,
+)
 
 from django.db.backends.base.base import BaseDatabaseWrapper
-from django.db.models import Q, QuerySet
+from django.db.models import Q
 from django.db.models.fields import Field
 from django.db.models.lookups import Lookup, Transform
 from django.db.models.query import QuerySet
 from django.db.models.sql.query import Query
 from django.db.models.sql.compiler import SQLCompiler
 from django.db.models.sql.compiler import _AsSqlType
 
+if sys.version_info < (3, 8):
+    from typing_extensions import Literal
+else:
+    from typing import Literal
+
 class SQLiteNumericMixin:
     def as_sqlite(
         self, compiler: SQLCompiler, connection: BaseDatabaseWrapper, **extra_context: Any
     ) -> _AsSqlType: ...
 
 _Self = TypeVar("_Self")
-
 _Numeric = Union[float, Decimal]
 
 class Combinable: