input: Write a python function to implement a Stack data structure in Python.
  assert stack.push(1) == None
  assert stack.push(2) == None
  assert stack.push(3) == None
  assert stack.peek() == 3
  assert stack.pop() == 3
  assert stack.peek() == 2
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.is_empty():
              return self.items.pop()
          return None
  
      def peek(self):
          if not self.is_empty():
              return self.items[-1]
          return None
  
      def is_empty(self):
          return len(self.items) == 0
  
  # Example usage:
  stack = Stack()