write_shared_index(): use tempfile module

diff --git a/RelNotes b/RelNotes
index 4567ad64fe794a21e1d041ddf766c55cb2c1fc7a..3295d667f396a686891335ceaaf817545a7d9e25 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.7.7.3.txt
\ No newline at end of file
+Documentation/RelNotes/2.5.0.txt
\ No newline at end of file