[ui][registration][admin] Move __unicode__ to __str__

__unicode__ is a leftover of Python 2

beat/web/ui/registration/admin.py

@@ -32,7 +32,7 @@ from .models import PreregistrationProfile
 
 
 class RegistrationAdmin(admin.ModelAdmin):
-    list_display = ("__unicode__", "activation_key_expired")
+    list_display = ("__str__", "activation_key_expired")
     search_fields = ("user__username", "user__first_name")