make WatchableFSMixin instances picklable, by discarding watchers
make WatchableFSMixin instances picklable, by discarding watchers

Konubinix/pyfilesystem,piranna/pyfilesystem,rivenz/pyfilesystem,flying-circus/pyfilesystem,zopyx/pyfilesystem,PyFilesystem/pyfilesystem,pmitros/pyfs,pombreda/pyfilesystem,pmitros/pyfs