text: Title: static code to generated models
  Body: Hey there, for the maintainers, thanks for that great library.
  I would appreciate a heads-up on something I'm trying to do, which is basically adding some static code to a generate model.
  I'm not sure Jinja would be suitable for that, since it's just a templating. Can someone give me a direction on the best approach for that?
  For example, for an Enum class:
  
  ```python
  class Foo(str, Enum):
      foo = 'foo'
      bar = 'bar'
  ```
  
  I would like the generated model to override a special method:
  
  ```python
  class Foo(str, Enum):
      foo = 'foo'
      bar = 'bar'
      
      @classmethod
      def _missing_(cls, value):
          pass
  ```
  
  I'm generating my models from an `openapi.yml` spec. Appreciate any thoughts or help!
  