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/+/10381
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>