`pillow` seems mandatory for now - should we add it to the install requirements ?

[smarie]

See this log error : https://github.com/smarie/python-yamlable/runs/5073104330?check_suite_focus=true#step:12:46

INFO     -  generating gallery...

generating gallery for /home/runner/work/python-yamlable/python-yamlable/docs/generated/gallery... :   0%|          | 0/2 [00:00<?, ?it/s]
generating gallery for /home/runner/work/python-yamlable/python-yamlable/docs/generated/gallery... :   0%|          | 0/2 [00:00<?, ?it/s]
ERROR    -  Could not import pillow, which is required to rescale images (e.g., for thumbnails): No module named 'PIL'

[smarie]

if we can remove it for minimal builds not requiring any image resize it would be appreciated, too..