Can't process folders

[mwermelinger]

An error in the handling of paths in check_folder() means that folders can't be processed anymore:

• version 1.2 on PyPI crashes when calling endswith(".py") on a Path object
• version 1.2.1 reports 'file not found' because open() gets the basename, not full name.

To do:

• [x] do str(path) instead of path.name
• [x] adds tests that process a folder, not just an individual sample file