improve type conversion of SPI_processed in Python

Here is a patch to improves how PL/Python deals with very large values
of SPI_processed. The previous code converts anything that does not fit
into a C long into a Python float. But Python long has unlimited
precision, so we should be using that instead. And in Python 3, int and
long as the same, so there is no need to deal with any variations anymore.

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Peter Eisentraut http://www.2ndQuadrant.com/
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