Add PEP 629 to the Simple repository API spec page

Author: brettcannon

source/specifications/simple-repository-api.rst

@@ -8,4 +8,5 @@ Simple repository API
 The current interface for querying available package versions and
 retrieving packages from an index server is defined in :pep:`503`,
 with the addition of "yank" support (allowing a kind of file deletion)
-as defined in :pep:`592`.
+as defined in :pep:`592` and specifying the interface version provided
+by an index server in :pep:`629`.