Change <Model>.from_dict() to a @classmethod

[packyg]

Is your feature request related to a problem? Please describe.
I want to be able to extend a model generated by this project in my business logic. A basic example would be:

# GeneratedModel generated by openapi-python-client

class EnhancedGeneratedModel(GeneratedModel):
  def id_eq(self, other: GeneratedModel) -> bool:
     return self.id == other.id

I cannot use EnhancedGeneratedModel.from_dict as that would return a GeneratedModel.

Describe the solution you'd like
Update the from_dict template in openapi_python_client/templates/model.pyi to be a @classmethod instead of a @staticmethod and use the passed class object to instantiate the model.

i.e. change from

@staticmethod
def from_dict(d: Dict[str, Any]) -> "{{ model.reference.class_name }}":
  # ... 
  return {{ model.reference.class_name }}(...)

to

@classmethod
def from_dict(cls, d: Dict[str, Any]) -> "{{ model.reference.class_name }}":
  # ... 
  return cls(...)

Describe alternatives you've considered
Manually creating a from_dict method on the extended class.

[dbanty]

Sure, I don't see a problem with that. I do think we need to switch the return type to a TypeVar to have the types be more accurate.

I'm a bit slow on OSS dev for the next couple months, so feel free to send a PR if you want it faster.