Admin page cannot render horizontal radio buttons

[Musen]

To reproduce the bug, suppose we have the model Person defined in models.py

class Person(Models.model):
    age = CharField(choices = (('Y', 'Young'), ('O', 'Old')))

We want to display the choice of age in our admin change page in radio buttons, so we have the following codes in admin.py

from .models import Person

class PersonAdmin(admin.ModelAdmin):
    radio_fields = {'age': admin.HORIZONTAL}

admin.site.register(Person, PersonAdmin)

Then, we will get the vertical radio buttons instead of the horizontal ones.

The bug is caused by the wrong HTML attribute that the ModelAdmin uses to produce horizontal radio buttons. The correct attribute is radio-inline but ModelAdmin uses radiolist inline. Also, since HTML uses the vertical radio buttons as default, function get_ul_class is useless, and it can be simply replaced by an if statement.