input: Write a python function to implement a stack with push, pop, and peek methods.
  ```python
  class Stack():
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
     
      def pop(self):
          if len(self.items) == 0: 
              return None
          else: 
              return self.items.pop()
      
      def peek(self):
          if len(self.items) == 0:
              return None
          else:
              return self.items[-1]
  ```
  assert s := Stack(); s.push(1); s.push(2); s.push(3); s.pop() == 3
  assert s := Stack(); s.push(1); s.push(2); s.push(3); s.peek() == 3
  assert s := Stack(); s.push(1); s.push(2); s.pop() == 2
output: class Stack():
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
     
      def pop(self):
          if len(self.items) == 0: 
              return None
          else: 
              return self.items.pop()
      
      def peek(self):
          if len(self.items) == 0:
              return None
          else:
              return self.items[-1]