lxml 6.0.0 support

[zhemaituk]

lxml 6.0.0 has been released (https://pypi.org/project/lxml/).
Relax dependencies constraints to allow this version.
Unit tests pass with lxml==6.0.0, so it appears to be a drop in replacement

[chedrik]

@adbar any chance you could cut a release with this new change? I'm having some dependency resolution woes due to this and being able to utilize the new commit and use lxml 6.0.0 would be 💯 🥺

[zhemaituk]

second that. @adbar a GA release would be nice if possible.