Change pathlist type to better match dataset handlers

[cpupower]

The type for the pathlist parameter as used by e.g. ImageReader and LabelReader is currently specified as List[str], but since the type in the dataset handlers has changed to a frozenset, this should be changed accordingly.

For example we could use Iterable, Sized/Container or Collection, depending on what is necessary.

Please note, that the type may be extended in the future, e.g. as a Union type of the path iterable type and a future ImagesetCollection type.

See also:

• https://docs.python.org/3/library/typing.html