text: Title: Unusable password generator for Django
  Body: #### The problem
  
  The recently added `Password` generator for Django is helpful, but it's not clear how to use it to create an unusable password (similar to calling `set_unusable_password` on the generated user).
  
  #### Proposed solution
  
  Django's `set_unusable_password` is a call to `make_password` with `None` as the password argument:
  
  https://github.com/django/django/blob/0b506bfe1ab9f1c38e439c77b3c3f81c8ac663ea/django/contrib/auth/base_user.py#L118-L120
  
  Using `password = factory.django.Password(None)` will actually work (and will allow factory users to override the password if desired). However, currently the password argument to this factory is documented as a string and this option is not mentioned.
  
  #### Extra notes
  
  The default value of the `password` argument to `factory.django.Password` could also be set to `None`. This would make that factory generate unusable passwords by default, which may or may not be desired.
  