Using Python's copy instead of Django's copycompat since it was deprecated

Hi there,

As you can see here ```django.utils.copycompat``` was deprecated with Django 1.6 and Python's ```copy``` needs to be used instead:

https://docs.djangoproject.com/en/1.6/internals/deprecation/

I've just made that little change in two files so we can use ```django-xml``` with the current versions of Django.

Regards,

Antonio Ognio
Lima-Peru