Tests: switch to explicit u"unicode" literals

Drop `from __future__ import unicode_literals`;
it was there for Python 3.2 compatibility (which
Anymail doesn't support). Ensures tests use normal
strs in Python 2.x.

tests/test_postmark_integration.py

@@ -1,5 +1,3 @@
-from __future__ import unicode_literals
-
 import unittest
 
 from django.test import SimpleTestCase