Silence annoying translation error message in test.

Running the test_widget.TestDatePicker:test_date_validate_supported_locales_12_29_2020 test resulted in this message being printed to stderr:
```
/home/awood/devel/subscription-manager/src/subscription_manager/gui/widgets.py:526: GtkWarning: Whoever translated calendar:MY did so wrongly.

  self._calendar = gtk.Calendar()
```

This patch suppresses that message.  See https://bugzilla.redhat.com/show_bug.cgi?id=868441