Using query expression in model Meta.ordering causes AttributeError in admin

[Gabriel Amram]

When using the newly introduced query-expression ordering feature in either the model's Meta.ordering or in its ModelAdmin causes errors.

Steps to reproduce:
First variant - using the model's Meta.ordering option -

//models.py
class Game(models.Model):
    score = models.IntegerField(null=True)

     class Meta:
         ordering = [F("score").desc(nulls_last=True)]
...
// admin.py
admin.site.register(Game)

Accessing the admin site will now give an AttributeError: 'OrderBy' object has no attribute 'startswith'

Variant 2 - Using the ModelAdmin ordering attribute:

//admin.py
class GameAdmin(models.ModelAdmin):
    ordering = [F("score").desc(nulls_last=True)]

runserver fails with error: TypeError: argument of type 'OrderBy' is not iterable