Add lxml as a requirement

[JST5000]

Whenever I try to follow the steps on xrpl-dev-portal to install Dactyl, the initial dactyl_build fails with the error Couldn't find a tree builder with the features you requested: xml. Do you need to install a parser library?. until I install lxml - I think that means that lxml should be in the requirements for the tool, although maybe I'm wrong.

[JST5000]

(An alternate solution for my specific problem is to update the xrpl-dev-portal CONTRIBUTING page with an additional step to install lxml - https://github.com/XRPLF/xrpl-dev-portal/blob/master/CONTRIBUTING.md)