input: Write a function that initializes and creates a basic blockchain in Python using only the core modules. The function should return an instance of the `Blockchain` class with a newly created block containing a nonce and a previous hash value.
  
  assert create_blockchain("1", "00000000") == <__main__.Blockchain object at 0x7f8b9c0f3d90>
  assert create_blockchain("5", "abcdef12") == <__main__.Blockchain object at 0x7f8b9c0f3e10>
output: def create_blockchain(nonce, previous_hash):
      class Block:
          def __init__(self, nonce, previous_hash):
              self.nonce = nonce
              self.previous_hash = previous_hash
  
      class Blockchain:
          def __init__(self):
              self.chain = []
              self.create_block(nonce, previous_hash)
  
          def create_block(self, nonce, previous_hash):
              block = Block(nonce, previous_hash)
              self.chain.append(block)
  
      blockchain = Blockchain()
      return blockchain