Issue1955

When special methods (e.g. __add__ or __sub__) return NotImplemented, then operations which internally use these methods should raise appropriate TypeError exceptions.
In CPython:
>>> class A:
... def __add__(self, other):
... return NotImplemented
... def __sub__(self, other):
... return NotImplemented
... 
>>> A() + A()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'instance' and 'instance'
>>> A() - A()
Traceback (most recent call last): 
 File "<stdin>", line 1, in <module> 
TypeError: unsupported operand type(s) for -: 'instance' and 'instance' 
>>>
In Jython:
>>> class A:
... def __add__(self, other):
... return NotImplemented
... def __sub__(self, other):
... return NotImplemented
... 
>>> A() + A()
NotImplemented
>>> A() - A()
NotImplemented
>>>

This only affects old style classes

The problem here is that PyInstance#__add__ does not follow the contract of PyObject#__add__ in that it must return null when a user defined add impl returns NotImplemented
Currently PyInstance#instance___add__ defers to invoke_ex to do the work. However there's an invoke_ex_richcmp method that is doing the right thing (returning null for NotImplemented). Either that method should be used instead (and more appropriately renamed) or maybe invoke_ex should just adopt that logic.
Note that this problem affects likely every binary op in PyInstance, not just add (which is one reason why invoke_ex might have to adopt the logic).
So this fix could use a few more tests other than add. You *might* be able to recycle some of test_descr_jy.BinopCombinationsTestCase for this (but there's no need to go overboard)