Patch #2477: Added from __future__ import unicode_literals

The new PyParser_*Ex() functions are based on Neal's suggestion and initial patch. The new __future__ feature makes all '' and r'' unicode strings. b'' and br'' stay (byte) strings.
diff --git a/Include/code.h b/Include/code.h
index 0e89b88..8c00700 100644
--- a/Include/code.h
+++ b/Include/code.h
@@ -49,6 +49,7 @@
 #define CO_FUTURE_ABSOLUTE_IMPORT 0x4000 /* do absolute imports by default */
 #define CO_FUTURE_WITH_STATEMENT  0x8000
 #define CO_FUTURE_PRINT_FUNCTION  0x10000
+#define CO_FUTURE_UNICODE_LITERALS 0x20000
 
 /* This should be defined if a future statement modifies the syntax.
    For example, when a keyword is added.