Fix compatibility

This commit fixes usage of the Singleton metaclass so that it is
compatible with both Python 2 and Python 3.

dotbot/messenger/messenger.py

@@ -1,11 +1,10 @@
 import sys
 from ..util.singleton import Singleton
+from ..util.compat import with_metaclass
 from .color import Color
 from .level import Level
 
-class Messenger(object):
-    __metaclass__ = Singleton
-
+class Messenger(with_metaclass(Singleton, object)):
     def __init__(self, level = Level.LOWINFO):
         self.set_level(level)
 

dotbot/util/compat.py

@@ -0,0 +1,5 @@
+def with_metaclass(meta, *bases):
+    class metaclass(meta):
+        def __new__(cls, name, this_bases, d):
+            return meta(name, bases, d)
+    return type.__new__(metaclass, 'temporary_class', (), {})