Fills bucket with accrued tokens since last fill.

def _fill(self):
        """Fills bucket with accrued tokens since last fill."""

        right_now = time.time()
        time_diff = right_now - self._last_fill
        if time_diff < 0:
            return

        self._count = min(
            self._count + self._fill_rate * time_diff,
            self._capacity,
        )
        self._last_fill = right_now