Update calendar to show the next month.

def _next_month(self):
        """Update calendar to show the next month."""
        self._canvas.place_forget()

        year, month = self._date.year, self._date.month
        self._date = self._date + self.timedelta(
            days=calendar.monthrange(year, month)[1] + 1)
        self._date = self.datetime(self._date.year, self._date.month, 1)
        self._build_calendar()