Continue rolling back pep-3141 changes that changed behavior from 2.5. This

round included:
 * Revert round to its 2.6 behavior (half away from 0).
 * Because round, floor, and ceil always return float again, it's no
   longer necessary to have them delegate to __xxx___, so I've ripped
   that out of their implementations and the Real ABC. This also helps
   in implementing types that work in both 2.6 and 3.0: you return int
   from the __xxx__ methods, and let it get enabled by the version
   upgrade.
 * Make pow(-1, .5) raise a ValueError again.

Lib/numbers.py

@@ -189,25 +189,6 @@ class Real(Complex):
         """
         raise NotImplementedError
 
-    @abstractmethod
-    def __floor__(self):
-        """Finds the greatest Integral <= self."""
-        raise NotImplementedError
-
-    @abstractmethod
-    def __ceil__(self):
-        """Finds the least Integral >= self."""
-        raise NotImplementedError
-
-    @abstractmethod
-    def __round__(self, ndigits=None):
-        """Rounds self to ndigits decimal places, defaulting to 0.
-
-        If ndigits is omitted or None, returns an Integral, otherwise
-        returns a Real. Rounds half toward even.
-        """
-        raise NotImplementedError
-
     def __divmod__(self, other):
         """divmod(self, other): The pair (self // other, self % other).