dataclasses.InitVar not supported

[pfrederiks]

Bug

When a dataclass contains an InitVar field, pydantic will try to validate it and does not pass the InitVar to post_init.

Using InitVar will raise the following error:

RuntimeError: no validator found for <class 'dataclasses.InitVar'>

And when arbitrary_types_allowed is set the following exception is raised:

TypeError: _pydantic_post_init() takes 1 positional argument but 2 were given

I have a pull request.

For bugs/questions:

• OS: any
• Python version: 3.7
• Pydantic version : 0.28