fix: 'load_module()' deprecated in Python 3.12

Starting with Python 3.10 we get the following warning:

  <frozen importlib._bootstrap>:283: DeprecationWarning: the
  load_module() method is deprecated and slated for removal in
  Python 3.12; use exec_module() instead

Replace our use of 'load_module()' with the recommended [1] alternative
that includes 'module_from_spec()' introduced in Python 3.5. Since we
need to support Python 3.4, keep the original code with a version check.

[1] https://docs.python.org/3.12/library/importlib.html#importlib.machinery.SourceFileLoader.load_module

Change-Id: I77d14f14fcc39e0227baad959c3e211aba309a21
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11583
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11704
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>