pgsql: PL/Python: Make tests pass with Python 3.5

PL/Python: Make tests pass with Python 3.5

The error message wording for AttributeError has changed in Python 3.5.
For the plpython_error test, add a new expected file. In the
plpython_subtransaction test, we didn't really care what the exception
is, only that it is something coming from Python. So use a generic
exception instead, which has a message that doesn't vary across
versions.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f1b898759f4936e9185698e8624da832a99b933e

Modified Files
--------------
src/pl/plpython/expected/README | 1 +
src/pl/plpython/expected/plpython_error_5.out | 428 ++++++++++++++++++++
.../plpython/expected/plpython_subtransaction.out | 12 +-
.../expected/plpython_subtransaction_0.out | 8 +-
.../expected/plpython_subtransaction_5.out | 8 +-
src/pl/plpython/sql/plpython_subtransaction.sql | 4 +-
6 files changed, 445 insertions(+), 16 deletions(-)