input: Write a function to implement a hash table in Python that supports insert, delete, and retrieve operations. The hash table should be initialized with an array of size 256 and use chaining to handle collisions. The insert method should add a key-value pair to the hash table, the delete method should remove a key-value pair, and the retrieve method should return the value associated with a given key.
  
  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 hash_table.retrieve("key1") == "value1"
  assert hash_table.retrieve("key2") == "value2"
  assert hash_table.delete("key1") == None
  assert hash_table.retrieve("key1") == None
output: 
  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]
  