import-debian-package: stop using a PosixPath object as context manager

Traceback (most recent call last):
  File "/builds/pkg/apertis-dev-tools/tests/import-debian-package/../../tools/import-debian-package", line 681, in <module>
    main(sys.argv)
    ~~~~^^^^^^^^^^
  File "/builds/pkg/apertis-dev-tools/tests/import-debian-package/../../tools/import-debian-package", line 666, in main
    do_import(args)
    ~~~~~~~~~^^^^^^
  File "/builds/pkg/apertis-dev-tools/tests/import-debian-package/../../tools/import-debian-package", line 396, in do_import
    with ensure_dir_or_none(args.cache_dir) or TemporaryDirectory() as downloaddir:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'PosixPath' object does not support the context manager protocol