input: Write a function to implement a hash table in Python that supports insert, delete, and retrieve operations. The hash table should handle collisions using chaining.
  
  class HashTable:
      def __init__(self):
          self.table = [[] for _ in range(256)]
   
      def insert(self, key, value):
          h = hash(key) % 256
          for i, kv in enumerate(self.table[h]):
              if kv[0] == key:
                  self.table[h][i] = (key, value)
                  return
          self.table[h].append((key, value))
   
      def retrieve(self, key):
          h = hash(key) % 256
          for kv in self.table[h]:
              if kv[0] == key:
                  return kv[1]
   
      def delete(self, key):
          h = hash(key) % 256
          for i, kv in enumerate(self.table[h]):
              if kv[0] == key:
                  del self.table[h][i]
  
  assert HashTable().insert(1, 'a') == None
  assert HashTable().insert(257, 'b') == None
  assert HashTable().retrieve(1) == 'a'
  assert HashTable().retrieve(257) == 'b'
  assert HashTable().delete(1) == None
  assert HashTable().retrieve(1) == None
  assert HashTable().delete(257) == None
  assert HashTable().retrieve(257) == None
output: 
  def __init__(self):
      self.table = [[] for _ in range(256)]
  
  def insert(self, key, value):
      h = hash(key) % 256
      for i, kv in enumerate(self.table[h]):
          if kv[0] == key:
              self.table[h][i] = (key, value)
              return
      self.table[h].append((key, value))
  
  def retrieve(self, key):
      h = hash(key) % 256
      for kv in self.table[h]:
          if kv[0] == key:
              return kv[1]
  
  def delete(self, key):
      h = hash(key) % 256
      for i, kv in enumerate(self.table[h]):
          if kv[0] == key:
              del self.table[h][i]
  