Make env-setup work on Python 3
protect PluginLoader._extra_dirs from appending None

PluginLoader.add_directory() can receive None from, for example, 
Inventory.add_directory(self.basedir()) if host_list is a custom list.
None has no reasonable interpretation other than ignore it.
Merge pull request #1900 from bostrick/devel

protect PluginLoader._extra_dirs from appending None