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

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

mercurial/exthelper.py

@@ -325,7 +325,7 b' class exthelper:'
             # Required, otherwise the function will not be wrapped
             uisetup = eh.finaluisetup
 
-            @eh.wrapfunction(discovery, b'checkheads')
+            @eh.wrapfunction(discovery, 'checkheads')
             def wrapcheckheads(orig, *args, **kwargs):
                 ui.note(b'His head smashed in and his heart cut out')
                 return orig(*args, **kwargs)