I'm wondering what there is left to do here.  Are we close to being able 
to close this issue?  Some thoughts:

(1) I think the docs could use a little work (just expanding, and giving 
a few examples).  But it doesn't seem urgent that this gets done before 
the next round of alphas.

(2) Looking at it, I'm really not sure that implementing 
Rational.__format__ is worthwhile;  it's not obvious that we need a way
to format a Rational as a float.  So I'm -0 on this.  If others think 
Rational should have a __format__ method I'm still happy to implement 
it.

Is there much else that needs to be done here?