Trees don't follow symlink unless explicitly enabled

[uellue]

Watching a tree containing a symlink to '/' adds a watch to the whole file system, which is probably unwanted. To avoid such behavior by default, following symlinks has to be enabled explicitly. The code may still be vulnerable to TOCTOU.

Further changes:

• Use os.walk() to traverse the tree to watch
• Remove duplicate add_watch() for IN_MOVED_TO
• flake8
• Warn instead of failing hard if adding a watch in a tree fails. Can be, for example, a permission error.

[uellue]

About the older Python versions: Are they still relevant?