Require lxml if tweety doesn't work without it

[dgw]

Per #221 (comment) there are errors in tweety that can be solved by installing the lxml package.

That should probably be one of tweety's direct requirements, if it's needed to make things work properly. My own testing of existing synchronous code upgraded from 1.x to 2.0.2 showed that all the basics (that I use) Just Work still, but only if I manually install lxml in the environment. Without lxml I only get errors like "Exception: invalid response".