py3: add PY23() macro to switch string literal depending on python version

I have no better idea to work around the bytes-unicode divergence of
Py_BuildValue(). Maybe we can write a code transformer for C extensions? :)

mercurial/cext/util.h

@@ -14,6 +14,13 b''
 #define IS_PY3K
 #endif
 
+/* helper to switch things like string literal depending on Python version */
+#ifdef IS_PY3K
+#define PY23(py2, py3) py3
+#else
+#define PY23(py2, py3) py2
+#endif
+
 /* clang-format off */
 typedef struct {
 	PyObject_HEAD