wrapfunction: use sysstr instead of bytes as argument in "bookflow"

This is as valid and simpler, it will help us to eventually get ride of
`safehasattr`.

hgext/bookflow.py

@@ -117,8 +117,8 b' def cwd_at_bookmark(repo, mark):'
 
 
 def uisetup(ui):
-    extensions.wrapfunction(bookmarks, b'update', bookmarks_update)
-    extensions.wrapfunction(bookmarks, b'addbookmarks', bookmarks_addbookmarks)
+    extensions.wrapfunction(bookmarks, 'update', bookmarks_update)
+    extensions.wrapfunction(bookmarks, 'addbookmarks', bookmarks_addbookmarks)
     extensions.wrapcommand(commands.table, b'commit', commands_commit)
     extensions.wrapcommand(commands.table, b'pull', commands_pull)
     if not ui.configbool(MY_NAME, b'enable-branches'):