Resolve RemovedInDjango40Warning warnings

* gql/graphene-django/graphene_django/tests/test_converter.py:175:
RemovedInDjango40Warning: django.utils.translation.ugettext_lazy() is
deprecated in favor of django.utils.translation.gettext_lazy().

* graphene-django/graphene_django/utils/utils.py:28:
RemovedInDjango40Warning: force_text() is deprecated in favor of
force_str().

graphene_django/tests/models.py

@@ -1,7 +1,7 @@
 from __future__ import absolute_import
 
 from django.db import models
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 CHOICES = ((1, "this"), (2, _("that")))
 

graphene_django/tests/test_converter.py

@@ -1,7 +1,7 @@
 import pytest
 from collections import namedtuple
 from django.db import models
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from graphene import NonNull
 from py.test import raises
 

graphene_django/utils/utils.py

@@ -2,7 +2,7 @@ import inspect
 
 from django.db import models
 from django.db.models.manager import Manager
-from django.utils.encoding import force_text
+from django.utils.encoding import force_str
 from django.utils.functional import Promise
 
 from graphene.utils.str_converters import to_camel_case
@@ -25,7 +25,7 @@ def isiterable(value):
 
 def _camelize_django_str(s):
     if isinstance(s, Promise):
-        s = force_text(s)
+        s = force_str(s)
     return to_camel_case(s) if isinstance(s, str) else s