mpz type should support formatting types for float's, like int

E.g.:
```pycon
>>> from gmpy2 import mpz
>>> format(mpz(123), 'f')
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    format(mpz(123), 'f')
    ~~~~~~^^^^^^^^^^^^^^^
ValueError: Invalid conversion specification
>>> format(int(123), 'f')
'123.000000'
```