Clarified a sentence in the Python 3 docs.

- input and output streams are always :class:`bytes` objects.

Specifically, :attr:`HttpResponse.content <django.http.HttpResponse.content>`

contains :class:`bytes`, which may require refactoring your tests.This won't

be an issue if you use :meth:`~django.test.TestCase.assertContains` and

:meth:`~django.test.TestCase.assertNotContains`: these methods expect a

unicode string.

contains :class:`bytes`, which may become an issue if you compare it with a

:class:`str` in your tests. The preferred solution is to rely on

:meth:`~django.test.TestCase.assertContains` and

:meth:`~django.test.TestCase.assertNotContains`. These methods accept a

response and a unicode string as arguments.

Coding guidelines

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