Fix: imageio bug

[af-a]

Description

Running recon-surf.sh when the BF-corrected image does not already exist currently fails due to the following error from N4_bias_correct.py:

Traceback (most recent call last):
  File "/FastSurfer/recon_surf//N4_bias_correct.py", line 27, in <module>
    import image_io as iio
  File "/FastSurfer/recon_surf/image_io.py", line 106, in <module>
    ) -> Union[sitk.Image, tuple[sitk.Image, Any]]:
TypeError: 'type' object is not subscriptable
Command exited with non-zero status 1

This PR fixes the bug which causes this (which seems to have been introduced in #308) in the function declaration: using the typing module's Tuple instead of tuple.

Tested on one 1mm case.