Allow arithmetic operation between gmpy2 numbers and custom objects.

[vinklein]

Currently gmpy2's arithmetic operators support python numbers:

>>> from gmpy2 import mpz
>>> mpz(1) + 1
mpz(2)

But does not support objects with custom __mpz__, __mpq__, __mpfr__ or __mpc__ methods :

>>> class A:
...     def __mpz__(self):
...             return mpz(1)
... 
>>> a=A()
>>> mpz(1) + a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'mpz' and 'instance'

This issue has been first seen in sage's ticket 23052.

I am currently working on a proposal for this issue.