ui: fix configbytes isinstance check to look for bytes and not str stable

ui: fix configbytes isinstance check to look for bytes and not str

Fixes configbytes on Python 3.

--- a/mercurial/ui.py	Mon Jul 24 14:42:55 2017 -0400
+++ b/mercurial/ui.py	Mon Jul 24 13:50:25 2017 -0400
@@ -648,7 +648,7 @@
             if default is _unset:
                 default = 0
             value = default
-        if not isinstance(value, str):
+        if not isinstance(value, bytes):
             return value
         try:
             return util.sizetoint(value)