Add stub for typing.ForwardRef (#3876)

* Add stub for class ForwardRef

* Add anotation for localns

* PEP8 and add version info check

stdlib/3/typing.pyi

@@ -659,3 +659,18 @@ def NewType(name: str, tp: Type[_T]) -> Type[_T]: ...
 
 # This itself is only available during type checking
 def type_check_only(func_or_cls: _C) -> _C: ...
+
+if sys.version_info >= (3, 7):
+    from types import CodeType
+    class ForwardRef:
+        __forward_arg__: str
+        __forward_code__: CodeType
+        __forward_evaluated__: bool
+        __forward_value__: Optional[Any]
+        __forward_is_argument__: bool
+        def __init__(self, arg: str, is_argument: bool = ...) -> None: ...
+        def _evaluate(self, globalns: Optional[Dict[str, Any]],
+                      localns: Optional[Dict[str, Any]]) -> Optional[Any]: ...
+        def __eq__(self, other: Any) -> Union[bool, NotImplemented]: ...
+        def __hash__(self) -> int: ...
+        def __repr__(self) -> str: ...