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.

Back-patch of commit f16d522, which
was previously back-patched into 9.2-9.4, but missed 9.5.

Author: tglsfdc

src/pl/plpython/expected/README

@@ -1,6 +1,7 @@
 Guide to alternative expected files:
 
 plpython_error_0.out			Python 2.4 and older
+plpython_error_5.out			Python 3.5 and newer
 
 plpython_unicode.out			server encoding != SQL_ASCII
 plpython_unicode_3.out			server encoding == SQL_ASCII