pgsql: In hstore_plpython, avoid crashing when return value isn't a map

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

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/85ec8bcce2608b8e29a1a0742282d39b29b78dda

Modified Files
--------------
contrib/hstore_plpython/expected/hstore_plpython.out | 11 +++++++++++
contrib/hstore_plpython/hstore_plpython.c            |  8 +++++++-
contrib/hstore_plpython/sql/hstore_plpython.sql      | 11 +++++++++++
3 files changed, 29 insertions(+), 1 deletion(-)