In hstore_plpython, avoid crashing when return value isn't a mapping.

Python 3 changed the behavior of PyMapping_Check(), breaking the
test in plpython_to_hstore() that verifies whether a function result
to be transformed is acceptable.  A backwards-compatible fix is to
first verify that the object doesn't pass PySequence_Check().

Perhaps accidentally, our other uses of PyMapping_Check() already
follow uses of PySequence_Check(), so that no other bugs were
created by this change.

Per bug #17908 from Alexander Lakhin.  Back-patch to all supported
branches.

Dmitry Dolgov and Tom Lane

Discussion: https://postgr.es/m/17908-3f19a125d56a11d6@postgresql.org

contrib/hstore_plpython/sql/hstore_plpython.sql

@@ -27,6 +27,17 @@ $$;
 SELECT test1n('aa=>bb, cc=>NULL'::hstore);
 
 
+-- test that a non-mapping result is correctly rejected
+CREATE FUNCTION test1bad() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+return "foo"
+$$;
+
+SELECT test1bad();
+
+
 -- test hstore[] -> python
 CREATE FUNCTION test1arr(val hstore[]) RETURNS int
 LANGUAGE plpython3u