I don't think any of the python modules should truly be considered "optional." Sure, Sigil could run without them, but as Kevin mentioned, those are the modules that are being packaged with Sigil (and eventually, all three platforms will have a package). Plugin developers will not be required to "declare" any of those modules as needing to be installed to use their plugin. It will be assumed that any Sigil installation will have access to all of those. At the very least, they should be considered "Most Strenuously Recommended If You Want Help Figuring Out Why A Particular Downloaded Plugin Isn't Working for You."
I can guarantee my first trouble-shooting step will be to require that the Test Plugin passes every single test before proceeding.
