For development I had setup a couple of plugins to be symlinks to the plugin directory checked out of git. E.g. in my `~/.config/MusicBrainz/Picard/plugins` I had entires like:

lrwxrwxrwx 1 phw users 59 25. Jan 2015 fanarttv -> /home/phw/devel/musicbrainz/picard-plugins/plugins/fanarttv
lrwxrwxrwx 1 phw users 77 27. Okt 2015 papercdcase.py -> /home/phw/devel/musicbrainz/picard-plugins/plugins/papercdcase.py

The symlinks to directories will cause issues when upgrading and restarting Picard:

Traceback (most recent call last):
  File "/usr/bin/picard", line 2, in <module>
    from picard.tagger import main; main('/usr/share/locale', True)
  File "/usr/lib/python3.6/site-packages/picard/tagger.py", line 776, in main
    tagger = Tagger(picard_args, unparsed_args, localedir, autoupdate)
  File "/usr/lib/python3.6/site-packages/picard/tagger.py", line 207, in _init_
    self.pluginmanager.load_plugindir(USER_PLUGIN_DIR)
  File "/usr/lib/python3.6/site-packages/picard/plugin.py", line 267, in load_plugindir
    self.remove_plugin(name)
  File "/usr/lib/python3.6/site-packages/picard/plugin.py", line 369, in remove_plugin
    shutil.rmtree(dirpath)
  File "/usr/lib/python3.6/shutil.py", line 490, in rmtree
    onerror(os.path.islink, path, sys.exc_info())
  File "/usr/lib/python3.6/shutil.py", line 488, in rmtree
    raise OSError("Cannot call rmtree on a symbolic link")
OSError: Cannot call rmtree on a symbolic link

I will submit a patch.