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.
    
    Resolves: #1959080