hgweb: use sysstr to get attribute on diff option

Attribute identifier should be `str` not `bytes`.

mercurial/hgweb/webcommands.py

@@ -1050,7 +1050,9 b' def annotate(web):'
             }
 
     diffopts = webutil.difffeatureopts(web.req, web.repo.ui, b'annotate')
-    diffopts = {k: getattr(diffopts, k) for k in diffopts.defaults}
+    diffopts = {
+        k: getattr(diffopts, pycompat.sysstr(k)) for k in diffopts.defaults
+    }
 
     return web.sendtemplate(
         b'fileannotate',