Fix force_str deprecation warning (#858)

Author: zhelyabuzhsky

graphene_django/converter.py

@@ -1,6 +1,6 @@
 from collections import OrderedDict
 from django.db import models
-from django.utils.encoding import force_text
+from django.utils.encoding import force_str
 
 from graphene import (
     ID,
@@ -30,7 +30,7 @@
 
 
 def convert_choice_name(name):
-    name = to_const(force_text(name))
+    name = to_const(force_str(name))
     try:
         assert_valid_name(name)
     except AssertionError: