Support references to non-object types

[p1-alexandrevaney]

Describe the bug
root component schemas do not get parsed correctly, resolved to None

To Reproduce
Steps to reproduce the behavior:
openapi-python-client generate --path path_to_test.yaml

Expected behavior
The generated model Link should have href: Union[Unset, str] = UNSET as attribute
instead we have href: Union[Unset, None] = UNSET

OpenAPI Spec File

openapi: 3.0.0

info:
  version: '1'
  title: 'test file'
  description: 'description'

paths: {}

components:
  schemas:
    Link:
      type: object
      properties:
        href:
          Uri:
            type: string

Desktop (please complete the following information):

• OS: Ubuntu 20.04
• Python Version: 3.8.5
• openapi-python-client version: up to date with main

Additional context

I am trying to build a 5g stack with the open source 5g api. This file in particular contains a lot of components schemas that get resolved to None

https://github.com/jdegre/5GC_APIs/blob/master/TS29571_CommonData.yaml

For example the Uri components is translated to

@attr.s(auto_attribs=True)
class Uri:
    """  """

    additional_properties: Dict[str, Any] = attr.ib(init=False, factory=dict)

which does not seem right as it is only a type:string component

[dbanty]

@p1-alexandrevaney I don't think I understand the example spec.

components:
  schemas:
    Link:
      type: object
      properties:
        href:
          Uri:
            type: string

Shouldn't type: string be directly part of the href property? What is Uri supposed to do?

[p1-alexandrevaney]

The real world example is this:

https://github.com/jdegre/5GC_APIs/blob/master/TS29571_CommonData.yaml#L296

    Link:
      type: object
      properties:
        href:
          $ref: '#/components/schemas/Uri'

It has a ref to https://github.com/jdegre/5GC_APIs/blob/master/TS29571_CommonData.yaml#L182

    Uri:
      type: string

So i figured it should be resolved to

    Link:
      type: object
      properties:
        href:
          Uri:
            type: string

A simpler example would be to just have

    Link:
      type: string

During the generation, the type=string will be ignored and the model will be

ModelProperty(name='Link', required=True, nullable=False, default=None, python_name='link', reference=Reference(class_name='Link', module_name='link'), required_properties=[], optional_properties=[], description='', relative_imports=set(), additional_properties=True)

[dbanty]

Ah, our reference resolution is pretty barebones right now and I think only supports references to objects (not basic types), so that's probably the issue.

[dbanty]

No one should try to do this until #366 is done though, since that big rework of reference resolution will conflict with anything else related.