Make tests work with python3

Author: Rory McCann

tests/pdfkit-tests.py

@@ -300,7 +300,7 @@ def test_raise_error_if_bad_wkhtmltopdf_option(self):
             r.to_pdf()
 
         raised_exception = cm.exception
-        self.assertRegexpMatches(raised_exception.message, '^wkhtmltopdf exited with non-zero code 1. error:\nUnknown long argument --bad-option\n')
+        self.assertRegexpMatches(str(raised_exception), '^wkhtmltopdf exited with non-zero code 1. error:\nUnknown long argument --bad-option\n')
 
 if __name__ == "__main__":
     unittest.main()