input: Write a python function to create a stack class with push, pop, peek, is_empty, and size methods.
  assert s := Stack(); s.push(10); s.push(20); s.size() == 2
  assert s.pop() == 20
  assert s.peek() == 10
  assert not s.is_empty()
  assert s.pop() == 10
  assert s.is_empty()
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          return self.items.pop() if not self.is_empty() else None
  
      def peek(self):
          return self.items[-1] if not self.is_empty() else None
  
      def is_empty(self):
          return len(self.items) == 0
  
      def size(self):
          return len(self.items)