docs: def add_directories(self, directories, except_blacklisted=True):
          """
          Adds `directories` to the set of plugin directories.
  
          `directories` may be either a single object or a iterable.
  
          `directories` can be relative paths, but will be converted into
          absolute paths based on the current working directory.
  
          if `except_blacklisted` is `True` all `directories` in
          `self.blacklisted_directories` will be removed
          """
          directories = util.to_absolute_paths(directories)
          if except_blacklisted:
              directories = self._remove_blacklisted(directories)
  
          self.plugin_directories.update(directories)