input: Write a python function to implement a stack with push, pop, and is_empty methods.
  assert __name__ == '__main__':
      s = Stack()
      assert s.is_empty() == True
      s.push(5)
      assert s.is_empty() == False
      assert s.pop() == 5
      assert s.is_empty() == True
      s.push(10)
      s.push(20)
      assert s.pop() == 20
      assert s.pop() == 10
      assert s.pop() == None
output: def Stack():
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.items:
              return None
          return self.items.pop()
  
      def is_empty(self):
          return len(self.items) == 0