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.