Fix plpython's overoptimistic caching of information about the rowtype of

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.)