Add stub for django.db.models.expressions.Expression.identity (#2685)

I don't think we need to be very specific about the contents of the
returned tuple; it's just used for comparison and hashing methods.

Author: cjwatson

django-stubs/db/models/expressions.pyi

@@ -118,7 +118,9 @@ class BaseExpression:
     def select_format(self, compiler: SQLCompiler, sql: str, params: _ParamsT) -> _AsSqlType: ...
     def get_expression_for_validation(self) -> BaseExpression: ...
 
-class Expression(_Deconstructible, BaseExpression, Combinable): ...
+class Expression(_Deconstructible, BaseExpression, Combinable):
+    @cached_property
+    def identity(self) -> tuple[Any, ...]: ...
 
 class CombinedExpression(SQLiteNumericMixin, Expression):
     connector: str

scripts/stubtest/allowlist_todo.txt

@@ -173,7 +173,6 @@ django.contrib.gis.db.models.DurationField.formfield
 django.contrib.gis.db.models.DurationField.get_db_converters
 django.contrib.gis.db.models.EmailField.formfield
 django.contrib.gis.db.models.Exists.empty_result_set_value
-django.contrib.gis.db.models.Expression.identity
 django.contrib.gis.db.models.ExpressionWrapper.allowed_default
 django.contrib.gis.db.models.Extent.is_extent
 django.contrib.gis.db.models.Extent3D.is_extent
@@ -520,7 +519,6 @@ django.db.models.DurationField.formfield
 django.db.models.DurationField.get_db_converters
 django.db.models.EmailField.formfield
 django.db.models.Exists.empty_result_set_value
-django.db.models.Expression.identity
 django.db.models.ExpressionWrapper.allowed_default
 django.db.models.Field.__copy__
 django.db.models.Field.__deepcopy__
@@ -662,7 +660,6 @@ django.db.models.expressions.Func.allowed_default
 django.db.models.expressions.When.allowed_default
 django.db.models.expressions.Col.relabeled_clone
 django.db.models.expressions.Exists.empty_result_set_value
-django.db.models.expressions.Expression.identity
 django.db.models.expressions.OrderBy.as_oracle
 django.db.models.expressions.OrderBy.as_sql
 django.db.models.expressions.Ref.relabeled_clone