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 a fixed size of 256 buckets and handle key collisions using chaining. If the given key is not found during a retrieve or delete operation, the function should return None.
  
  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]
          return None
   
      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]
                  return
          return None
  
  assert HashTable().retrieve("missing_key") == None
  assert HashTable().delete("missing_key") == None
  hash_table = HashTable()
  hash_table.insert("key1", "value1")
  assert hash_table.retrieve("key1") == "value1"
  hash_table.insert("key1", "new_value")
  assert hash_table.retrieve("key1") == "new_value"
  hash_table.delete("key1")
  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]
          return None
   
      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]
                  return
          return None
  