input: Write a python function to add, delete, and list contacts using the ContactManager class.
  assert __name__ == "__main__" 
  contact_manager = ContactManager() 
  contact_manager.add_contact("Alice")
  contact_manager.add_contact("Bob") 
  contact_manager.add_contact("Charlie") 
  contact_manager.delete_contact("Bob") 
  assert contact_manager.contact_list == ["Alice", "Charlie"]
  contact_manager.list_contacts()
output: class ContactManager:
      def __init__(self):
          self.contact_list = []
  
      def add_contact(self, name):
          if name not in self.contact_list:
              self.contact_list.append(name)
  
      def delete_contact(self, name):
          if name in self.contact_list:
              self.contact_list.remove(name)
  
      def list_contacts(self):
          for contact in self.contact_list:
              print(contact)