Update lxml to work with python 3.14 (#21150)

Older lxml causes venv creation with python 3.14 build failure.
Latest lxml requires python 3.8 which is luckily also the current
requirement for llvm.
So it is safe to bump version.

Author: lslusarczyk

third_party/requirements.txt

@@ -13,7 +13,7 @@ exhale==0.3.0
 idna==3.7
 imagesize==1.1.0
 Jinja2==3.1.6
-lxml==4.9.3
+lxml==6.0.2
 Mako==1.3.0
 MarkupSafe==2.1.5
 packaging==24.2