Update dependency lxml to v4.9.1 [SECURITY]

Author: renovate[bot]

ML/requirements.txt

@@ -183,7 +183,7 @@ libcst==0.3.23
 libretranslatepy==2.1.1
 livereload==2.6.3
 lockfile==0.12.2
-lxml==4.6.5
+lxml==4.9.1
 Mako==1.1.6
 Markdown==3.3.6
 MarkupSafe @ file:///tmp/build/80754af9/markupsafe_1621528148836/work

requirements.txt

@@ -199,7 +199,7 @@ livereload==2.6.3
 llvmlite==0.36.0
 lockfile==0.12.2
 louis==3.12.0
-lxml==4.6.5
+lxml==4.9.1
 macaroonbakery==1.3.1
 Mako==1.1.6
 Markdown==3.3.6