ui: explicitly open config files in binary mode

This has been working mostly accidentally, but now it works explicitly.

mercurial/ui.py

@@ -176,7 +176,7 b' class ui(object):'
     def readconfig(self, filename, root=None, trust=False,
                    sections=None, remap=None):
         try:
-            fp = open(filename)
+            fp = open(filename, u'rb')
         except IOError:
             if not sections: # ignore unless we were looking for something
                 return

tests/test-check-py3-commands.t

@@ -9,6 +9,6 b' The full traceback is hidden to have a s'
   >   $PYTHON3 `which hg` $cmd 2>&1 2>&1 | tail -1
   > done
   version
-  TypeError: startswith first arg must be str or a tuple of str, not bytes
+  TypeError: cannot use a string pattern on a bytes-like object
   debuginstall
-  TypeError: startswith first arg must be str or a tuple of str, not bytes
+  TypeError: cannot use a string pattern on a bytes-like object