Make all fields readonly for Django Admin

admin.py

class OpsIPInfoAdmin(admin.ModelAdmin):

    def get_readonly_fields(self, request, obj=None):
        # make all fields readonly
        readonly_fields = list(
            set([field.name for field in self.opts.local_fields] +
                [field.name for field in self.opts.local_many_to_many])))

        if 'is_submitted' in readonly_fields:
            readonly_fields.remove('is_submitted')

        return readonly_fields