adda bit about __class__ and subclassing to metaprogramming materials

[PythonCHB]

In the Circle exercise, folks created methods that were not that friendly to subclassing.

use of the __class__ attribute could help with this:

In [61]: class C: 
    ...:     def test_name(self): 
    ...:         print("class name is:") 
    ...:         print(self.__class__.__name__) 
    ...:                                                                        

In [62]: c = C()                                                                

In [63]: c.test_name()                                                          
class name is:
C

similarly (and more importantly) for things like add, etc.

some notes and examples of this in the metaprogramming materials would be a good idea.