PL/Python: Simplify PLyLong_FromInt64

PL/Python: Simplify PLyLong_FromInt64

We don't actually need two code paths, one for 32 bits and one for 64
bits.  Since the existing code already assumed that "long long" is
available, we can just use PyLong_FromLongLong() for 64 bits as well.
In Python 2.5 and later, PyLong_FromLong() and PyLong_FromLongLong() use
the same code, so there will be no difference for 64-bit platforms.  In
Python 2.4, the code is different, but performance testing showed no
noticeable difference in PL/Python, and that Python version is ancient
anyway.

Discussion: https://www.postgresql.org/message-id/0a02203c-e157-55b2-464e-6087066a1849@2ndquadrant.com