pgsql: Fix plpython's overoptimistic caching of information about the

Log Message:
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Fix plpython's overoptimistic caching of information about the rowtype of
a trigger's target table.  The rowtype could change from one call to the
next, so cope in such cases, while avoiding doing repetitive catalog lookups.
Per bug #3847 from Mark Reid.

Backpatch to 8.2.x.  Likely this fix should go further back, but I can't test
it because I no longer have a machine with a pre-2.5 Python installation.
(Maybe we should rethink that idea about not supporting Python 2.5 in the
older branches.)

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/pl/plpython:
        plpython.c (r1.90.2.3 -> r1.90.2.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.90.2.3&r2=1.90.2.4)