ui: fix opts labeling on ui.warn et al for Python 3

This is a step towards fixing extension load warnings on Python
3. Note that I suspect there are still some bugs in this area and that
things like color won't work, but the code at least executes and
prints text to the console correctly now.

mercurial/ui.py

@@ -1213,14 +1213,14 b' class ui(object):'
         This adds an output label of "ui.status".
         '''
         if not self.quiet:
-            opts['label'] = opts.get('label', '') + ' ui.status'
+            opts[r'label'] = opts.get(r'label', '') + ' ui.status'
             self.write(*msg, **opts)
     def warn(self, *msg, **opts):
         '''write warning message to output (stderr)
 
         This adds an output label of "ui.warning".
         '''
-        opts['label'] = opts.get('label', '') + ' ui.warning'
+        opts[r'label'] = opts.get(r'label', '') + ' ui.warning'
         self.write_err(*msg, **opts)
     def note(self, *msg, **opts):
         '''write note to output (if ui.verbose is True)
@@ -1228,7 +1228,7 b' class ui(object):'
         This adds an output label of "ui.note".
         '''
         if self.verbose:
-            opts['label'] = opts.get('label', '') + ' ui.note'
+            opts[r'label'] = opts.get(r'label', '') + ' ui.note'
             self.write(*msg, **opts)
     def debug(self, *msg, **opts):
         '''write debug message to output (if ui.debugflag is True)
@@ -1236,7 +1236,7 b' class ui(object):'
         This adds an output label of "ui.debug".
         '''
         if self.debugflag:
-            opts['label'] = opts.get('label', '') + ' ui.debug'
+            opts[r'label'] = opts.get(r'label', '') + ' ui.debug'
             self.write(*msg, **opts)
 
     def edit(self, text, user, extra=None, editform=None, pending=None,