write_shared_index(): use tempfile module

diff --git a/RelNotes b/RelNotes
index 53d76d1a47e0750b0d8b14287ff3d923ca55b438..3295d667f396a686891335ceaaf817545a7d9e25 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.7.12.1.txt
\ No newline at end of file
+Documentation/RelNotes/2.5.0.txt
\ No newline at end of file