Fix conversions.py2to3 on unicode strings (#67)

The introduction of string kinds in #49 missed updating the 2to3 visitor
for it. Update the `visit_Str` case to pass `s.kind` to `Str`.

Fixes #66.

Author: msullivan

typed_ast/conversions.py

@@ -218,7 +218,7 @@ def visit_Str(self, s):
         if isinstance(s.s, bytes):
             return ast3.Bytes(s.s)
         else:
-            return ast3.Str(s.s)
+            return ast3.Str(s.s, s.kind)
 
     def visit_Num(self, n):
         new = self.generic_visit(n)