Add validator to PL/Python

src/include/catalog/catversion.h

src/include/catalog/pg_pltemplate.h

src/pl/plpython/expected/README

src/pl/plpython/expected/plpython_error.out

src/pl/plpython/expected/plpython_error_0.out

src/pl/plpython/expected/plpython_record.out

src/pl/plpython/expected/plpython_types.out

src/pl/plpython/expected/plpython_types_3.out

src/pl/plpython/plpython.c

src/pl/plpython/sql/plpython_error.sql