old_contents: from django.contrib.auth import forms, get_user_model
  from django.core.exceptions import ValidationError
  from django.utils.translation import ugettext_lazy as _
  
  User = get_user_model()
  
  
  class UserChangeForm(forms.UserChangeForm):
      class Meta(forms.UserChangeForm.Meta):
          model = User
  
  
  class UserCreationForm(forms.UserCreationForm):
  
      error_message = forms.UserCreationForm.error_messages.update(
          {"duplicate_username": _("This username has already been taken.")}
      )
  
      class Meta(forms.UserCreationForm.Meta):
          model = User
  
      def clean_username(self):
          username = self.cleaned_data["username"]
  
          try:
              User.objects.get(username=username)
          except User.DoesNotExist:
              return username
  
          raise ValidationError(self.error_messages["duplicate_username"])
  
new_contents: from django.contrib.auth import forms, get_user_model
  from django.core.exceptions import ValidationError
  from django.utils.translation import gettext_lazy as _
  
  User = get_user_model()
  
  
  class UserChangeForm(forms.UserChangeForm):
      class Meta(forms.UserChangeForm.Meta):
          model = User
  
  
  class UserCreationForm(forms.UserCreationForm):
  
      error_message = forms.UserCreationForm.error_messages.update(
          {"duplicate_username": _("This username has already been taken.")}
      )
  
      class Meta(forms.UserCreationForm.Meta):
          model = User
  
      def clean_username(self):
          username = self.cleaned_data["username"]
  
          try:
              User.objects.get(username=username)
          except User.DoesNotExist:
              return username
  
          raise ValidationError(self.error_messages["duplicate_username"])
  
subject: Update ugettext_lazy to gettext_lazy