py3: use unicodes in __slots__

__slots__ in Python 3 accepts only unicodes and there is no harm using
unicodes in __slots__. So just adding u'' is fine. Previous occurences of this
problem are treated the same way.

mercurial/statprof.py

@@ -177,7 +177,7 b' state = ProfileState()'
 class CodeSite(object):
     cache = {}
 
-    __slots__ = ('path', 'lineno', 'function', 'source')
+    __slots__ = (u'path', u'lineno', u'function', u'source')
 
     def __init__(self, path, lineno, function):
         self.path = path
@@ -232,7 +232,7 b' class CodeSite(object):'
         return os.path.basename(self.path)
 
 class Sample(object):
-    __slots__ = ('stack', 'time')
+    __slots__ = (u'stack', u'time')
 
     def __init__(self, stack, time):
         self.stack = stack

tests/test-check-py3-compat.t

@@ -36,7 +36,6 b''
   mercurial/cffi/mpatch.py: error importing: <ImportError> No module named 'mercurial.cffi' (error at check-py3-compat.py:*)
   mercurial/cffi/osutil.py: error importing: <ImportError> No module named 'mercurial.cffi' (error at check-py3-compat.py:*)
   mercurial/scmwindows.py: error importing: <ImportError> No module named 'msvcrt' (error at win32.py:*)
-  mercurial/statprof.py: error importing: <TypeError> __slots__ items must be strings, not 'bytes' (error at statprof.py:*)
   mercurial/win32.py: error importing: <ImportError> No module named 'msvcrt' (error at win32.py:*)
   mercurial/windows.py: error importing: <ImportError> No module named 'msvcrt' (error at windows.py:*)