input: Write a python function to calculate the area of a circle given its radius using a class.
  assert Circle(2).area() == 12.56
  assert Circle(5).area() == 78.5
  assert Circle(0).area() == 0
output: import math
  
  class Circle:
      def __init__(self, radius):
          self.radius = radius
      
      def area(self):
          return round(math.pi * self.radius ** 2, 2)