No obvious way to set self.maxDiff with pytest_django.asserts

[r-thomson]

When using pytest_django.asserts.assertXMLEqual, the diff in the console is truncated, with the following message at the end:

Diff is ### characters long. Set self.maxDiff to None to see it.

It's not obvious to me what the right way to accomplish this with pytest-django is, since the TestCase for these functions is defined internally. Right now I'm just monkeypatching unittest.case.TestCase.maxDiff, but it'd be nice to have an obvious way of doing this.