drop the `lxml` dependency

[fabi1cazenave]

lxml raises two issues :

• the lack of type annotations can confuse mypy ;
• it’s the only dependency for Lafayette-style installer scripts.

Dropping it in favor of the standard XML API would be a plus. I can’t even remember why I’ve used lxml in the first place.

[fabi1cazenave]

Good first step before addressing #121.