Em dash

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Author: hugovk

run_release.py

@@ -100,8 +100,8 @@
    so it's worth checking back even after reading earlier versions.
 
 
-Summary -- release highlights
-=============================
+Summary --- release highlights
+==============================
 
 .. This section singles out the most important changes in Python {version}.
    Brevity is key.