ForwardRef stub for python 3.6 raises error on MyPy check

[SirTelemak]

Bug

#1598 introduce stub class for ForwardRef without __init__ for type checking so mypy raise error for ForwardRef
https://github.com/samuelcolvin/pydantic/blob/e985857e5a9ede8d346b010a5a039aa84a089826/pydantic/typing.py#L31-L33
I not pretty sure is this is planned or bug and if this is a bug I can fix it with stub __init__

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

pydantic version: 1.6.1
            pydantic compiled: False
                 install path: \venv\Lib\site-packages\pydantic
               python version: 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 22:45:29) [MSC v.1916 32 bit (Intel)]
                     platform: Windows-10-10.0.18362-SP0
     optional deps. installed: ['typing-extensions', 'email-validator']

test.py

# For python version < 3.7
from pydantic.typing import ForwardRef

ref = ForwardRef('A')

class A:
    ...

mypy test.py
> test.py:3: error: Too many arguments for "ForwardRef"