Fix: Windows DLL path lookup with Python >= 3.8

Starting with Python 3.8 on Windows [1] the DLLs lookup when loading
native modules was changed to not search PATH like everything else on
this platform. Restore this behavior by doing it manually.

Simon adds: it only started to matter now because MSYS2 dropped a local
patch [2] that retained the old behaviour.

[1]: https://docs.python.org/3/library/os.html#os.add_dll_directory
[2]: https://www.msys2.org/news/#2023-08-06-python-changed-behavior-when-loading-dll-dependencies-of-extension-modules

Change-Id: Id436244a717f440db11c33b6d418f31c7838591d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11043