Support ForwardRef in Python 3.6

Currently pydantic only supports ForwardRef in Python 3.7. It will be great if it can also be supported in Python 3.6. Although a ForwardRef can’t be explicitly created in Python 3.6, it can be implicitly created:

class A(BaseModel):
    a: 'A' = None

If you take a look at the ForwardRef class in Python 3.6 and Python 3.7, they share a very similar interface. The only major difference I spotted is that in python 3.6, ForwardRef is named as _ForwardRef and the method _evaluate in python 3.7 is named as _eval_type in python 3.6.

Quickly looking through the code I believe we can change https://github.com/samuelcolvin/pydantic/blob/a704662ae4cde638c0e1b68126a2f89828787641/pydantic/utils.py#L24-L28 to

try: 
     from typing import ForwardRef  # type: ignore 
 except ImportError: 
     # python 3.6 
    from typing import _ForwardRef as ForwardRef

and https://github.com/samuelcolvin/pydantic/blob/a704662ae4cde638c0e1b68126a2f89828787641/pydantic/main.py#L456-L459 to

 for f in cls.__fields__.values(): 
     if type(f.type_) == ForwardRef: 
         f.type_ = f.type_._eval_type(globalns, localns or None)  # type: ignore 
         f.prepare() 

[mooncake4132]

Thanks. I’ll prepare a PR once https://github.com/samuelcolvin/pydantic/pull/464 lands.

[samuelcolvin]

Done.