Fix name of Whats New file (#127)

Author: JelleZijlstra

run_release.py

@@ -960,7 +960,7 @@ def maybe_prepare_new_main_branch(db: DbfilenameShelf) -> None:
 
     prev_branch = f"{release_tag.major}.{release_tag.minor}"
     new_branch = f"{release_tag.major}.{int(release_tag.minor)+1}"
-    whatsnew_file = f"Doc/whatsnew/{new_branch}"
+    whatsnew_file = f"Doc/whatsnew/{new_branch}.rst"
     with cd(db["git_repo"]), open(whatsnew_file, "w") as f:
         f.write(WHATS_NEW_TEMPLATE.format(version=new_branch, prev_version=prev_branch))