ui: fix implicit unicode-to-bytes conversion introduced in 9df29b7c62cf

This is harmless, unless you try and run hg with HGUNICODEPEDANTRY
enabled. It's technically wrong, so let's go ahead and fix it.

Differential Revision: https://phab.mercurial-scm.org/D3989

mercurial/ui.py

@@ -392,7 +392,7 b' class ui(object):'
     def readconfig(self, filename, root=None, trust=False,
                    sections=None, remap=None):
         try:
-            fp = open(filename, u'rb')
+            fp = open(filename, r'rb')
         except IOError:
             if not sections: # ignore unless we were looking for something
                 return