Fixed bug in experiment_unittests

[tjk584]

There was a problem with pkgutil.itermodules where passing a Path() object worked for certain python versions, but not others. Passing a string works for all versions.

The error this bugfix fixes was:

Traceback (most recent call last):
  File "/usr/lib64/python3.10/pkgutil.py", line 417, in get_importer
    importer = sys.path_importer_cache[path_item]
KeyError: PosixPath('/home/radar/borealis/src/borealis_experiments')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/radar/borealis//tests/experiments/experiment_unittests.py", line 372, in <module>
    run_tests(sys.argv[1:])
  File "/home/radar/borealis//tests/experiments/experiment_unittests.py", line 332, in run_tests
    build_experiment_tests(experiments, args.kwargs)
  File "/home/radar/borealis//tests/experiments/experiment_unittests.py", line 249, in build_experiment_tests
    for (_, name, _) in pkgutil.iter_modules([Path(experiment_path)]):
  File "/usr/lib64/python3.10/pkgutil.py", line 129, in iter_modules
    for i in importers:
  File "/usr/lib64/python3.10/pkgutil.py", line 421, in get_importer
    importer = path_hook(path_item)
  File "<frozen importlib._bootstrap_external>", line 1632, in path_hook_for_FileFinder
  File "<frozen importlib._bootstrap_external>", line 1504, in __init__
  File "<frozen importlib._bootstrap_external>", line 182, in _path_isabs
AttributeError: 'PosixPath' object has no attribute 'startswith'

See python/cpython#88227 for more info

[Doreban]

Seems good to me