Fix aux tests for Python 3.5

I used the `encoding` param for `run()` which is for Python 3.6+. For
the moment I removed it so as to keep support for Python 3.5. Thanks
Travis CI.

passphrase/tests/tests_aux.py

@@ -182,12 +182,11 @@ def test_print_stderr(self):
                     string
                 )
             ],
-            encoding='UTF-8',
             stdout=PIPE,
             stderr=PIPE,
         )
-        self.assertEqual(proc.stdout, '')
-        self.assertEqual(proc.stderr, string + '\n')
+        self.assertEqual(proc.stdout.decode('utf-8'), '')
+        self.assertEqual(proc.stderr.decode('utf-8'), string + '\n')
         # The following is just for coverage, the actual test is above
         Aux.print_stderr('')