pgsql: Fix subtransaction test for Python 3.10

Fix subtransaction test for Python 3.10

Starting with Python 3.10, the stacktrace looks differently:
- PL/Python function "subtransaction_exit_subtransaction_in_with", line 3, in <module>
- s.__exit__(None, None, None)
+ PL/Python function "subtransaction_exit_subtransaction_in_with", line 2, in <module>
+ with plpy.subtransaction() as s:
Using try/except specifically makes the error look always the same.

(See https://github.com/python/cpython/pull/25719 for the discussion
of this change in Python.)

Author: Honza Horak <hhorak(at)redhat(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/853083.1620749597%40sss.pgh.pa.us
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1959080

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9c31e4165522eb510123a05597f541dadff4e679

Modified Files
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src/pl/plpython/expected/plpython_subtransaction.out | 11 +++++++----
src/pl/plpython/sql/plpython_subtransaction.sql | 7 +++++--
2 files changed, 12 insertions(+), 6 deletions(-)