Add lxml to install_requires

Author: orlnub123

docs/setup.py

@@ -4,7 +4,7 @@
 setup(
     name="docs",
     packages=find_packages(),
-    install_requires=["mkdocs~=1.0", "markdown<3", "py-gfm"],
+    install_requires=["mkdocs~=1.0", "markdown<3", "py-gfm", "lxml"],
     entry_points={
         "mkdocs.plugins": ["index_generator = plugins.generators:IndexGeneratorPlugin"]
     },