pgsql: Simplify some SPI tests of PL/Python

Simplify some SPI tests of PL/Python

These tests relied on both next() and __next__(), but only the former is
needed since Python 2 support has been removed, so let's simplify a bit
the tests.

Author: Erik Wienhold
Discussion: /messages/by-id/173209043143.2092749.13692266486972491694@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db80507d98462d95f2f7f8c1a58e55bb6337c3eb

Modified Files
--------------
src/pl/plpython/expected/plpython_spi.out | 23 +++++------------------
src/pl/plpython/sql/plpython_spi.sql | 23 +++++------------------
2 files changed, 10 insertions(+), 36 deletions(-)