Removed unnecessary dep from tox.ini

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

tox.ini

diff --git a/tox.ini b/tox.ini
index 43456c6..ec7d104 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,7 +14,6 @@
     # readme_renderer
     flake8
     pytest
-    lxml
 commands =
     check-manifest --ignore tox.ini,tests*
     # This repository uses a Markdown long_description, so the -r flag to