XMLRenderer tries to import StringIO from rest_framework.compat

[brandoncazander]

When trying to use XMLRenderer, an ImportError is raised:

ImportError: cannot import name StringIO

The custom StringIO has been removed from rest_framework.compat.

[jpadilla]

@brandoncazander yeah good catch. Can you put a PR together to fix it? If not I'll try to get to it later today or tomorrow.