PSD_tools v1.8 breaking changes

Hi all,

From version v1.8 PSD_tools has breaking changes: some functions are renamed, so PSD preview does not work with latest PSD_tools package.

See: https://psd-tools.readthedocs.io/en/latest/migration.html#psdimage
And: https://psd-tools.readthedocs.io/en/latest/migration.html#pil-export

My fix: https://github.com/haiwen/seahub/pull/4775