Fixed #18269 -- Applied unicode_literals for Python 3 compatibility.

Thanks Vinay Sajip for the support of his django3 branch and
Jannis Leidel for the review.

docs/ref/forms/fields.txt

@@ -25,8 +25,6 @@ exception or returns the clean value::
     >>> f = forms.EmailField()
     >>> f.clean('foo@example.com')
     u'foo@example.com'
-    >>> f.clean(u'foo@example.com')
-    u'foo@example.com'
     >>> f.clean('invalid email address')
     Traceback (most recent call last):
     ...