scmutil: clean up use of two-argument raise

This makes any attempt to port to Python 3 harder, and the new syntax
is supported in 2.4 already.

mercurial/scmutil.py

@@ -959,4 +959,4 b' class filecache(object):'
         try:
             del obj.__dict__[self.name]
         except KeyError:
-            raise AttributeError, self.name
+            raise AttributeError(self.name)