Exclude .DS_Store from local_backups cleanup

[davebuilt]

The app crashes when it encounters a file that isn't a directory inside the local_backups folder.

This is fine usually but on macOS the system occasionally inserts .DS_Store files.

You need to filter to just directories for the cleanup or specifically ignore .DS_Store files.

Traceback (most recent call last):
  File "PPP.py", line 542, in <module>
    backupLocal()
  File "PPP.py", line 380, in backupLocal
    'local_backups', backups[oldest_backup]))
  File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/shutil.py", line 494, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/shutil.py", line 413, in _rmtree_safe_fd
    onerror(os.scandir, path, sys.exc_info())
  File "/usr/local/Cellar/python/3.7.7/Frameworks/Python.framework/Versions/3.7/lib/python3.7/shutil.py", line 409, in _rmtree_safe_fd
    with os.scandir(topfd) as scandir_it:
NotADirectoryError: [Errno 20] Not a directory: 'local_backups/.DS_Store'

P.S. I love this tool, thanks for making it! If my Python was better I would submit a pull request.