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

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.

--- a/mercurial/ui.py	Fri Mar 03 12:55:11 2017 -0500
+++ b/mercurial/ui.py	Fri Mar 03 14:08:24 2017 -0500
@@ -1213,14 +1213,14 @@
         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 @@
         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 @@
         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,