input: Write a python function to calculate mean and variance of a list of numbers.
  assert calculate_mean_variance([1, 2, 3, 4, 5]) == (3.0, 2.0)
  assert calculate_mean_variance([10, 20, 30, 40, 50]) == (30.0, 200.0)
  assert calculate_mean_variance([2, 4, 4, 4, 5, 5, 7, 9]) == (5.0, 4.0)
output: def calculate_mean_variance(numbers):
      if not numbers:
          return (0, 0)
      mean = sum(numbers) / len(numbers)
      variance = sum((x - mean) ** 2 for x in numbers) / len(numbers)
      return (mean, variance)