Use PyFloat_Pack8() on Python 3.11 alpha7

[vstinner]

The private _PyFloat_Pack8() and _PyFloat_Unpack8() functions were removed in Python 3.11, but Python 3.11 alpha7 adds new clean public PyFloat_Pack8() and PyFloat_Unpack8() functions: see https://bugs.python.org/issue46906

• https://github.com/zopefoundation/zodbpickle/blob/master/src/zodbpickle/_pickle_33.c#L1695
• https://github.com/zopefoundation/zodbpickle/blob/master/src/zodbpickle/_pickle_33.c#L4247

zodbpickle should use them to be compatible with Python 3.11.

[icemac]

@vstinner Thank you for informing us. We'll use that functions when a7 is released.