Replace the remaining occurrences of six.u with the u prefix
diff --git a/tests/test_fernet.py b/tests/test_fernet.py
index 5b7a9f9..0b93f01 100644
--- a/tests/test_fernet.py
+++ b/tests/test_fernet.py
@@ -99,9 +99,9 @@
     def test_unicode(self, backend):
         f = Fernet(base64.urlsafe_b64encode(b"\x00" * 32), backend=backend)
         with pytest.raises(TypeError):
-            f.encrypt(six.u(""))
+            f.encrypt(u"")
         with pytest.raises(TypeError):
-            f.decrypt(six.u(""))
+            f.decrypt(u"")
 
     @pytest.mark.parametrize("message", [b"", b"Abc!", b"\x00\xFF\x00\x80"])
     def test_roundtrips(self, message, backend):