Make __getitem__ handle slice for Python 3

Python 3 doesn't have __getslice__ instead it uses __getitem__ with a
slice object.

Author: msabramo

smmap/buf.py

@@ -51,6 +51,8 @@ def __len__(self):
         return self._size
 
     def __getitem__(self, i):
+        if isinstance(i, slice):
+            return self.__getslice__(i.start or 0, i.stop or self._size)
         c = self._c
         assert c.is_valid()
         if i < 0: