Cannot overwrite 'get_fieldsets' in custom UserAdmin

I'm using a custom user model and a custom UserAdmin.

I try do overwrite the 'get_fieldsets' method to change some fields and hide some others, bud django admin still uses the  'get_fieldsets' method from django.contrib.auth.admin.UserAdmin.

My CustomUserAdmin Class:

from django.contrib.auth.admin import UserAdmin
from users.models import User  # custom user model


class CustomUserAdmin(UserAdmin):

    add_fieldsets = (
        (None, {
            'classes': ('wide',),
            'fields': ('email', 'username', 'password1', 'password2')
        }),
    )

    # The forms to add and change user instances
    form = UserChangeForm
    add_form = UserCreationForm

    # The fields to be used in displaying the User model.
    # These override the definitions on the base UserAdmin
    # that reference specific fields on auth.User.
    list_display = ('email', 'username', 'first_name', 'last_name', 'is_active', 'is_staff',
                    'is_superuser')
    list_filter = ('is_active', 'is_staff', 'is_superuser')
    search_fields = ('email', 'first_name', 'last_name', 'username')
    ordering = ('email',)
    filter_horizontal = ('groups', 'user_permissions',)


    def get_fieldsets(self, request, obj=None):
        if not obj:
            return self.add_fieldsets

        if request.user.is_superuser and request.user.pk != obj.pk:
            perm_fields = ('is_active', 'is_staff', 'is_superuser',
                           'groups', 'user_permissions')
        else:
            perm_fields = ('is_active', 'is_staff', 'groups')

        return [(None, {'fields': ('email', 'password')}),
                (_('Personal info'), {'fields': ('first_name', 'last_name', 'username', 'biography')}),
                (_('Permissions'), {'fields': perm_fields}),
                (_('Important dates'), {'fields': ('last_login', 'date_joined')})]


admin.site.register(User, CustomUserAdmin)

Some body have an idea, whats going on?

I'm working on this for days.

Please help.

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