[PyGreSQL] Return value for OIDs

[Christoph Zwerschke]

I noticed that in pgmodule.c in the function pgsource_oidstatus(),
the Oid is treated as a long in C and returned as an int in Python,
while in the function pg_query(), the oid is treated as a long long
in C and returned as a long in Python. I think we should handle this
consistently. I think we can safely assume that an Oid has 4 bytes.

-- Chris