lxml 6.0.0 removes support for Python < 3.8

https://github.com/lxml/lxml/blob/lxml-6.0.0/CHANGES.txt#L59
> * Support for Python < 3.8 was removed.

Drop py36 from test target list.

Author: qykth-git

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py38,py39,py310,py311,py312,py313,mypy
+envlist = py38,py39,py310,py311,py312,py313,mypy
 skipsdist = True
 
 [testenv]