PL/Python: Remove use of simple slicing API
PL/Python: Remove use of simple slicing API

The simple slicing API (sq_slice, sq_ass_slice) has been deprecated
since Python 2.0 and has been removed altogether in Python 3, so remove
those functions from the PLyResult class.  Instead, the non-slice
mapping functions mp_subscript and mp_ass_subscript can take slice
objects as an index.  Since we just pass the index through to the
underlying list object, we already support that.  Test coverage was
already in place.

adam8157/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb