fix: test_message_iterator.py hangs on Python 3.12

fix: test_message_iterator.py hangs on Python 3.12

Starting with Python 3.12, `None` is immortal: its refcount operations
are NO-OP and sys.getrefcount() will return a static value of UINT_MAX
on 64-bit and UINT_MAX >> 2 on 32-bit.

This basically transform `test_try_again_many_times` in an almost
infinite loop and hangs the testsuite.

Detect this by checking if the refcount on `None` is incremented after
assigning to a variable and skip the test if it's not the case.

See PEP-0683[1] for the gory details.

[1] https://peps.python.org/pep-0683/

Change-Id: Id07658245d524288ce7606cb0a011ad97068dad1
Signed-off-by: Michael Jeanson <mjeanson@debian.org>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10379
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>