py3: use unicode literals in changelog.py

collections.namedtuple type and field names must be str in Python 3.
Our custom module importer was rewriting them to bytes literals,
making this fail.

In addition, __slots__ values must also be unicode.

mercurial/changelog.py

@@ -138,9 +138,10 b' def _delayopener(opener, target, buf):'
         return appender(opener, name, mode, buf)
     return _delay
 
-_changelogrevision = collections.namedtuple('changelogrevision',
-                                            ('manifest', 'user', 'date',
-                                             'files', 'description', 'extra'))
+_changelogrevision = collections.namedtuple(u'changelogrevision',
+                                            (u'manifest', u'user', u'date',
+                                             u'files', u'description',
+                                             u'extra'))
 
 class changelogrevision(object):
     """Holds results of a parsed changelog revision.
@@ -151,8 +152,8 b' class changelogrevision(object):'
     """
 
     __slots__ = (
-        '_offsets',
-        '_text',
+        u'_offsets',
+        u'_text',
     )
 
     def __new__(cls, text):