Add FormattedValue and JoinedStr to ast for py3.6+ (#2206)

Fixes #2205

For reference https://docs.python.org/3/library/ast.html#abstract-grammar

stdlib/3/_ast.pyi

@@ -233,6 +233,15 @@ class Num(expr):
 class Str(expr):
     s = ...  # type: str
 
+if sys.version_info >= (3, 6):
+    class FormattedValue(expr):
+        value = ...  # type: expr
+        conversion = ...  # type: typing.Optional[int]
+        format_spec = ...  # type. typing.Optional[expr]
+
+    class JoinedStr(expr):
+        values = ...  # type: typing.List[expr]
+
 class Bytes(expr):
     s = ...  # type: bytes