Add future_builtins stub file (#606)

Add stub file for https://docs.python.org/2/library/future_builtins.html.

stdlib/2.7/future_builtins.pyi

@@ -0,0 +1,14 @@
+from typing import Any
+
+from itertools import ifilter as filter
+from itertools import imap as map
+from itertools import izip as zip
+
+
+def ascii(obj: Any) -> str: ...
+
+
+def hex(x: int) -> str: ...
+
+
+def oct(x: int) -> str: ...