Fix LGTM recommendation: Module is imported with 'import' and 'import from'

Module 'os' is imported with both 'import' and 'import from'

I feel we don't need this abbreviation here:
- Lines are not too long.
- os.path is known by all Python developpers, while the op abbreviation is
  not (I believe the osp abbreviation is more common).

And actually we had both os.path.exists() and op.join() in the code.
That's confusing.