Just had a quick look at numbers.py.  Only two comments:

1. I think there's a typo in the docstring for Inexact:  one of those == should be a != 

2. Not that it really matters now, but note that at least for Decimal, x-y is not the same 
as x+(-y)  (I'm looking at Complex.__sub__), and +x is not a no-op (Real.real, 
Real.conjugate).  In both cases signs of zeros can be problematic:

>>> x = Decimal('-0')
>>> y = Decimal('0')
>>> x-y
Decimal("-0")
>>> x+(-y)
Decimal("0")
>>> x
Decimal("-0")
>>> +x
Decimal("0")

Of course the first point wouldn't matter anyway since Decimal already implements __sub__;  
the second means that if Decimal were to join Real,  something would need to be done to 
avoid Decimal("-0").real becoming Decimal("0").