Memory leak in plpython3u (with testcase and patch)

Hi all,

I found a memory leak in plpython3u. It happens when converting the
argument for an spi plan execution (via plpy.execute() or similar). I've
attached a sql file to create two plpython3u functions to reproduce the
issue (memory_leak_test.sql).

When running
```
set client_min_messages = 'debug';
select test_mem(1000);
```
You'll see that the memory goes to 720MB.

In contrast the test_mem_query function you won't see the leak (it uses
~70MB):
```
set client_min_messages = 'debug';
select test_mem_query(1000);
```
This is because test_mem_query executes the query using a sql string
instead of a spi plan with parameters.

I believe the issue is in the call to `PLy_output_convert` inside
`PLy_spi_execute_plan`. I've attached a patch that reduces the memory usage
to ~70MB from ~720 MB when using test_mem. I based what I did on what
`PLy_input_convert` does. But I am not an expert in this part of the code
so I am not sure the patch is correct. In particular, I don't fully grasp
the comment in `PLy_input_convert` about why the scratch is reset before
not after the conversion cycle and what that has to do with python
refcounts. A review by an expert would be appreciated.

Thanks,
Mat

-- 
Mat Arye, Technical lead of all thing AI @ Timescale