swig-py: Fix return type of svn_fs_commit_txn() on Python 3.

* subversion/bindings/swig/svn_fs.i
  (%typemap(argout) (const char **conflict_p, svn_revnum_t *new_rev)):
    Use "bytes or None" conversion to build return value on Python 3.
Index: subversion/bindings/swig/svn_fs.i
===================================================================
--- subversion/bindings/swig/svn_fs.i	(revision 1880203)
+++ subversion/bindings/swig/svn_fs.i	(working copy)
@@ -108,7 +108,13 @@
     /* this is always Py_None */
     Py_DECREF($result);
     /* build the result tuple */
-    $result = Py_BuildValue("zi", *$1, (long)*$2);
+    $result = Py_BuildValue(
+%#if PY_VERSION_HEX >= 0x03000000
+                            "yi",
+%#else
+                            "zi",
+%#endif
+                            *$1, (long)*$2);
 }
 #endif